Hmm, not quite sure how to phrase this, but am I expecting too much from my players to interact with the characters and the world on their own?

So, my players recently returned back from a mission, and the guild they are part of threw them a party in celebration.

As such, I gave the players some time to freeroam and talk to the other member of the guild, but they kind of .. didn't. There was some surface level banter, sure, and one player asked about some NPC that got injured in a previous sessions. But other than that? Nothing.

Now, when I asked them later, one player in particular mentioned he didn't feel like talking to any NPCs, because he didn't have any attachment to them.

Except, that's what the party was there for. So the players can have some time to talk with the guild members they don't get to talk to often and to, through their interaction with them, build an attachment. Maybe ask them about their favorite food, the place they hang out at or what plans they have for next week. To get to know them on a personal level and build a relationship. But ... nothing.

So, according to my players they won't interact with an NPC in a meaningful way, if they aren't attached to them. However, as far as I am concerned, the only way they can grow attached to the NPC is through interacting with them first.

This is something I've noticed time and time again. Players just not asking any question past the most surface level stuff ever -I guess because they don't feel connected to the character-, and then they are confused why they don't feel connected to any characters.

I dunno. Maybe I'm the problem? Maybe I'm just asking too much of my players? I don't think so.

And quiet frankly, I'm not quiet sure what their expectation is either. How am I supposed to make a character meaningful, without the players interacting on more than a surface level with them first?

But what are you guys thinking?

Edit: to clarify some misconceptions. A lot of people assumed that I just threw the players into a room with a bunch of characters they've never met. And, while that is partially true for the players, as I mentioned in the beginning of my original post, every NPC present at the party were members of the same guild, and thus characters the PCs, at least based on their lore, have known and whom they grew up around for years.
However, the players never really interacted with those NPCs until now, since they began the game two weeks out from town on a hunt, with none of the guild NPCs anywhere near close by, until they returned and the guild held a party in their honor.

The players also of course had a basic profile for everyone, like "that's dave. he's a half-orc barbarian and the guild leader. he adopted [PC's name], after his parents died" or "that's abby. she's an elf and a former druid, but was kicked out of her circle and now tends to the guild's animals" or "that's todd. he's the tiefling's cleric and just recently officiated dave's and abby's wedding". 

Also most NPCs were clustered together in some groups, to help initiate role play, and as a jumping point of from where the players could jump into more meaningful role-play. Like a bard sitting on stage and playing music while another one sings, or the guilds artificer and the blacksmith tinkering with some loot someone brought in, or two rouges in a corner trying to see who has the fastest hands, by performing ever more difficult card tricks.

The goal wasn't to make them interact with some complete strangers, but give them a chance to interact with and build a connection to the NPCs their characters canonically already have a deep bond with.

Quote from Paefny >>

How am I supposed to make a character meaningful, without the players interacting on more than a surface level with them first?

Visibly connect the things the players do invest in to the NPC roleplay and engagement.

If the players focus on Monster of the Week or We Exist to Kill Things and Take Loot aspects of the game make it clear that they need to talk to NPC's about it.

"Oh, you want to talk to Old Bob about the Sunken River Palace. You can find him at the Lost Flagon most afternoons."

Bob Encounter

Bob isn't talkative and needs to be "warmed up" before he'll trust anyone with more info. A DC 15 Persuasion Check (roleplay and roll play it) will loosen him up. If the players buy Old Bob a drink, the check will be at advantage. Bob doesn't take threats well, any Intimidation Check is at DC 15 and will make him hostile.

Be explicit about the encounter and its mechanics. "You need to convince Bob to tell you more. This will require a Persuasion or Intimidation Check after some roleplay to open him up."

I use the Insect analogy.  Only 2 behaviors, approach or avoidance.  Why would a PC manifest approach behavior? 

They would have to know about a potential reward.  They would need a way to assess potential risk.  If the reward is within reach and the risk seems low, then they may formulate an approach. Any learned behavior has to happen in the context of an approach.  Learning to interact with NPCs has to happen as part of an approach to assess the risk/reward. 

Don't describe things that a PC can't interact with.  Highlight only what they can interact with and give them a reason to do so.

Yeah NPC interactions should advance some story in game-- even if the story is just the relationship between characters-- each interaction should have stakes and lead to something more. Players aren't interested in small talk, and while a DM may think the NPC's are all fascinating and heroic, players don't have a read on that unless it's conveyed to them through action. 

To really get your players interested in the NPC's, have the NPC's offer hooks to draw them in. Have them offer interesting tidbits about the world, have them gossip about which guild member once trained under a great wizard and knows where his lost treasure might be found. Show the players something they can DO with your NPC's, and then see which hook they're drawn to, THEN develope that relationship. 

What kind of game are your players looking for? Some players are just there to turn their brains off and roll dice to kill goblins. If they're just there for hack and slash while eating snacks and chilling with friends, maybe they just aren't interested in lore or roleplaying.

So try and find out what your players want, and figure out a way to show them something in your world which will attract their interest. I ran a Spelljammer Academy campaign a couple months ago, and one of the players mentioned that his Druid was interested in seeing strange life in space. So I had the ship, on its mission, travel past the planet Garden, a mass of planetoids connected with giant vines, complete with a picture in the chat. The ship didn't even stop there, just passed by in a scene only about one minute long. The player was enthralled.  Something as simple as that was very effective.

From my experience players rarely care about more than one or two NPCs so throwing them into a party situation will just be a case of lots of characters they don't care about instead of a chance to meet new people. Honestly, how often have you played games like Skyrim and talked to every person in a town? You talk to the one that's giving you the quest, ask nothing about their background, and leave. Also don't forget that although you think they're all interesting and different to your players they're all basically you with only tiny differences so why work to get to know them?

Instead of trying to make them care about an entire guild work on really close ties to just one or two, that way you can do a much better job of making them feel unique and get a much better chance of the players caring. Starting with a quirk helps them be memorable, there's a reason Boblin the Goblin is a D&D meme rather than Dave the Ordinary, and then build from there. Also don't be afraid to drop an NPC if your players don't respond to them, it's hard to predict what they'll latch onto