It's the same reason people pay hundreds of dollars for expensive tennis shoes. It's street cred.

I'll throw in my personal experience and what I've seen with my friends (who spend too much). Basically, it depends on how you approach it. Quite often these games are live service and free to play. This means that there is no initial buy-in yet you can log many many hours. Once you've played 100 hours for free, many people may start to feel a few things, such as: "This gun is getting a little bland, and I quite like those ones I've been seeing/picking up" or "I've put in this many hours for free, surely it's worth a couple of dollars" or even "I like the game, I may as well support the developers". These sorts of sentiments (along with patterns in-game) often lead to that first purchase which may be only $20. Of course, once you've made that first purchase, you've opened the gate. Play another 100 hours? What's another $20 more, right? Have leftover "coins"? Gotta use them up! So on and so forth.

I have "fallen" for these same tactics before, although I use that term loosely as I went in fully knowing what it was. But when you play a game a lot, changing your experience via cosmetics can actually be quite satisfying. It's the same reason we used to have this shit for free, it's because people like customising their experience or changing things up once they get bland. The difference is these days it's shoved in your face, has massive budgets, and you may get made fun of (however jokingly or not) for being a "default skin" guy.

Obviously Diablo here is paid, but much of the same feelings can apply I would imagine.

I've bought a few skins to support developers on games I like.

The whales, addicts though they may be, are absolutely part of the problem, but I think Joe Consumer is just as guilty when buying these titles and shrugging their shoulders.

Watching many go on to defend these practices on Steam and elsewhere is bewildering. AAA gaming Stockholm syndrome or something, must defend the skinner box.