Why doesn't everyone in a milestone-campaign multiclass as a barbarian for the extra HP? Even if they never Rage (which gives them Resistance to Slashing, Piercing, and Bludgeoning Damage and Advantage on Strength Checks and Strength Saving Throws), they get a ton of extra HP (which is really important at lower levels)?
In a milestone-levelling campaign, you won't even have a more difficult time gaining a single level as a multiclassed character like you would in a xp-levellling campaign.

I'm very new to D&D and feel like I just discovered some kind of trick or hack, and I don't understand why everyone doesn't multiclass to Barbarian, which makes me wonder what knowledge I'm either missing or misunderstanding?

Please explain this to me!

Well first of all, because it requires 13 strength and only barbarians, paladins, and some fighters and clerics are likely to have that much investment (other classes might, but they will be much less common).

Second, I don't know what you mean by multiclassing using milestone not being as slow as XP. Multiclass or not gain levels at the same rate in either case. I think you might have heard that multiclassing slows down class feature progression which is not based on XP and still true. For example, if you are a level 5 character wizard 4/barbarian 1, you gave up being able to cast fireball for that extra HP, and have yo wait till level 6 to be as strong as a non-multiclass is at level 5.

Speaking of the HP: Lastly, this gains at most 3 HP per level of investment (d6=4, d12=7). The tough feat alone gives 2 HP at level 1, plus 2 HP per character level regardless of class. If HP is a concern, this is a much better investment. The first 2 drawbacks aren't worth just 3 HP if there isn't another feature you plan to use.

Also, keep in mind that you only get 20 character levels total. So you can't be a level 20 wizard and a level 20 barbarian. Also, you can't rage and cast spells or concentrate on spells, so that makes it less beneficial for spellcasters as well. If you want the extra HP, buffing CON or taking the Tough feat are usually more popular solutions for many folks.

This is definitely a very "video games" mindset. Fast paced games where spending points in a different class or to boost some feature other than your primary stats can be a minor inconvenience with big payoffs down the line. But, unless you're playing with a DM who just builds combat encounter after combat encounter without much story to interact with in between, you're not going to just breeze through levels no matter what your leveling system is. A single level of Barbarian could mean you're going a month or more in real time without getting access to a core, really fun ability in your primary class.

That all said, for martial characters a single level dip into barbarian is incredibly useful many times. Minor HP Boost, access to Unarmored Defense (which, even if you're not going unarmored into combat regularly, at the very least if you get woken up in the middle of the night without your armor on you'll be fine), and a twice-a-day boost to Defense and damage is nothing to sneeze at.

In addition to the above posters comments, multiclassing in general delays your progression in your main class. One level in barbarian means waiting an extra level for feats, access to spells, access to all your class abilities. And in exchange you get, what 7 hp at most? That’s not going to be worth the trade.

Just get tough feat, resilience con, fighting initiate defense, and Con ASIs for a tougher PC.

If you want a multiclass, rogue barbarians are a cool build. Make sure to take at least 2 levels in barbarian though.

As others have said multiclassing has problems whether your gaining levels via EXP or milestones ( which are typically set at about the same points as you would have the EXP to level anyway). The only time the HP for barbarian are significantly better than for casters is at L1 where it is 12+con vs 6+con but then you loose most of the starting proficiencies etc for the casting class and end up a level behind on your spell progression. The concentration canceling and spellcasting blocking of a barbarian’s rage creates significant problems with most casting classes including classes like ranger that you might think could fit together well. There are a number of “traps” like this that early players often have to work through before the get a solid feel for multiclassing. 

because some of us don't care about creating an uber build. Plus even if you used Standard array you need two 13 or better in your stats. 

Taking Barbarian over whatever you want to play gives you at most 6 extra HP (12 + Con Mod for Barbarian, 6 + Con Mod for wizard).

It will also delay your class progression, so you get extra attacks and ability score increases later than the other players.

How exactly do you think multiclassing functions and what do you think the differences between milestone play and XP play are? If you tell us that we can probably figure out where the confusion comes from.

I agree with lostwhilefishing, there are so many avenues to explain why multiclassing barbarian is not always, in fact often not, a good idea to do… and gatorgirl, being new to the game, poses a reasonable question for the knowledge they have at the moment.… Asking them how multi classing functions and how they believe leveling up styles entail is likely the source of the confusion.

Quote from gatorgirl7563 >>

Why doesn't everyone in a milestone-campaign multiclass as a barbarian for the extra HP? Even if they never Rage (which gives them Resistance to Slashing, Piercing, and Bludgeoning Damage and Advantage on Strength Checks and Strength Saving Throws), they get a ton of extra HP (which is really important at lower levels)?
In a milestone-levelling campaign, you won't even have a more difficult time gaining a single level as a multiclassed character like you would in a xp-levellling campaign.

I'm very new to D&D and feel like I just discovered some kind of trick or hack, and I don't understand why everyone doesn't multiclass to Barbarian, which makes me wonder what knowledge I'm either missing or misunderstanding?

Please explain this to me!

 I kind of think you answered your own question in the bolded. The extra HP is really only relevant at low levels, which usually you are not at for very long (milestone or XP). So a few sessions into the campaign it pretty much becomes a wash.

Any d10 class would only gain 2hp which isn’t worth the downside of lost progression. I would say the same for d8 classes as well. Rogues delay sneak attack damage and number of skills(I think) if they start Barbarian  clerics, druids, bards delay spells, etc. 

It mainly depends on your build to say if it is worthwhile to dip Barbarian at 1st level.