The short version is that an enormous, multibillion dollar gambling industry has been built around Valve's item marketplace, and in particular around CS:GO skins. If that sounds completely insane and stupid, I'm with you, but it exists. Valve takes their typical cut off of all of these trades, and thus derives massive profits from it.

Here's the long version: https://peertube.gravitywell.xyz/videos/watch/a8e6d20c-3003-4b14-b9c4-cb6a25b238e7?isPeertubeContent=1

Mainly Team Fortress 2 and Counter Strike GO/2, cuz both of them have cosmetics with rarities obtained via what effectively amounts to lootboxes. In one sense they also have an out-of-game economy around these things where these items are traded for actual money

There is a massive secondary market for in-game items (primarily CS skins) that Valve refuses to combat or even officially acknowledge. Some of it is legitimate, some of it is literal lottery for children. And since every transaction takes place on Steam, they get a cut of that.

TF2 was the original gacha game.

Loot boxes in Counter-Strike

All their big multiplayer games have lootboxes and stuff like that.