Quote from Greenstone_Walker >>

Show, don't tell.

Have them encounter another pack of thieves, one who is proudly showing off some nice item. When asked where they got it, "Oh, yeah, we were robbing a house and Dobby here noticed a hidden door, we risked spending more time in the hour to investigate, and found this nice piece. Cool eh?"

Partially this.  Your players are mission oriented.  They go in, they get the thing, they get out.  This wasn't a theft they schemed up they were assigned.  So they think they're performing their assignment.  I've notice, particularly younger players getting the hang of the game, lean a bit on following instructions.  

I had a similar issue in DitA where the characters were basically press-ganged into suppressing a cult uprising.  After a couple of encounters I realized they thought they're charter was strictly to take out cultists.  So, I set up an encounter with another Flaming Fist member who shot the breeze with them and was surprised they weren't taking any "spoils" with them.  I had that NPC off camera run in with their patron/charter who requested a status update on their "investigation."  Satisfied with their work, their patron "reminded" them that a condition of their contract grants them spoils over any cultist property they encounter during their investigation.  So they started lining their pockets .. and then went overboard.  They have now filed with the city of Baldur's Gate claim deeds to both the cults HQ and the mansion of the cult's financier.  I let it go, they now have a Wayne Manor and a Bat Cave.

Quote from pavilionaire >>

Quote from Farling >>

As Xalthu said, money isn't particularly useful in D&D 5E.

However, if you want to reward your PCs then perhaps their recruiter can provide items as appropriate rewards for their successful mission(s).

 So why do my players want it so much?

 Because living in a capatalist society primes players to want money even if their characters can’t use it. 🙂

Quote from Xalthu >>

Quote from pavilionaire >>

Quote from Farling >>

As Xalthu said, money isn't particularly useful in D&D 5E.

However, if you want to reward your PCs then perhaps their recruiter can provide items as appropriate rewards for their successful mission(s).

 So why do my players want it so much?

 Because living in a capatalist society primes players to want money even if their characters can’t use it. 🙂

 What do you mean PCs can't use money??

Quote from Wysperra >>

Quote from Xalthu >>

Quote from pavilionaire >>

Quote from Farling >>

As Xalthu said, money isn't particularly useful in D&D 5E.

However, if you want to reward your PCs then perhaps their recruiter can provide items as appropriate rewards for their successful mission(s).

 So why do my players want it so much?

 Because living in a capatalist society primes players to want money even if their characters can’t use it. 🙂

 What do you mean PCs can't use money??

 There’s just not that much to buy. Slight armor upgrades are about the only expensive thing with prices listed. Weapons are all pretty cheap. Magic items according to RAW are not for sale. Lots of people put out guides for how to spend money in 5e on things like hirelings, but if it were obvious how to spend your money, people wouldn’t need a guide. Most expenditure of money seems to require some degree of homebrewing.

Mithril and Adamantine weapons/armor are not cheap. Heavy armor is not cheap.

I'm fair certain there is a published list on how much magic items cost.

Buying a Stronghold and Followers (tm Matt Colville) is not cheap.

Adamantine Armor is a magic item, and as such, according to RAW it can’t normally be bought or sold, and has no price listed.

Personally, I do make non-magical weapon and armor upgrades available in stores in large cities in my game, but this is homebrew. Published magic item prices are unofficial. It’s not impossible to spend money in 5e, it’s just hard, and requires the DM to do some research outside of official WotC sourcebooks.

Like pavillionaire said, there’s not much to buy — magic item prices are all homebrew, colville’s book is homebrew (high quality, but still homebrew). I guess you might deduct lifestyle expenses, but after your fighter/pally gets their plate armor, there’s not a lot to buy otherwise. Oh, actually, I guess there’s things like diamonds for raise dead and other spell components. 

I think the other reason could be that back in 1e 1gp=1 xp. So then you really wanted the money to help level up. In 3 and 4, there were magic item shops all over the place, and an arms race to have the right bonus by the right level to keep up with the math. So if your DM didn’t give that +2 neck slot item, you really needed to fill in the gap.
Now, it’s nice to have money, but you can pick up a sword at level 1 and effectively use that same sword at level 20. Unless you’re buying a boat or something there’s not really a lot of big ticket items. But money has always been a part of the game’s reward system and is kind of hard wired in, even if it’s usefulness has dipped in this edition. 

Basic Rules:

Adamantine Weapons

Adamantine is an ultrahard metal found in meteorites and extraordinary mineral veins. In addition to being used to craft adamantine armor, the metal is also used for weapons.

Melee weapons and ammunition made of or coated with adamantine are unusually effective when used to break objects. Whenever an adamantine weapon or piece of ammunition hits an object, the hit is a critical hit.

The adamantine version of a melee weapon or of ten pieces of ammunition costs 500 gp more than the normal version, whether the weapon or ammunition is made of the metal or coated with it.

Xanathar's:

Magic Item Price

Let's not forget that material components for high levels spells do get pretty pricy, like gemstone a few hundred GP gemstone pricey.

That cult headquarters my players claimed as "spoils"  (like actually filed a deed, appeared before a magistrate for an affidavit pursuant to the claim, and working with the city for permits to operate a business on premises) has a persistent and dangerous sewer gas build up problem.  The sewer guild is going to _milk_ them to fix that once they find out how much loot the party fell into "over the course of their sanctioned investigation."  I never thought I'd have "rich people problems" in my game, but there it is.

Quote from pavilionaire >>

Quote from Farling >>

As Xalthu said, money isn't particularly useful in D&D 5E.

However, if you want to reward your PCs then perhaps their recruiter can provide items as appropriate rewards for their successful mission(s).

 So why do my players want it so much?

 Because they are used to other editions where it was far more useful - i.e. magic shops!

Quote from Farling >>

Quote from pavilionaire >>

Quote from Farling >>

As Xalthu said, money isn't particularly useful in D&D 5E.

However, if you want to reward your PCs then perhaps their recruiter can provide items as appropriate rewards for their successful mission(s).

 So why do my players want it so much?

 Because they are used to other editions where it was far more useful - i.e. magic shops!

 Agreed, there is nothing wrong with putting a magic shop in your game.