Aren't most micro-transactions like those purely cosmetic?

Yes for Valve, but that hasn't stopped a secondary market transacting tens of thousands of dollars or more for them in some cases.

> https://dmarket.com/blog/most-expensive-csgo-skins

> https://tradeit.gg/csgo/store

They are in Valve's own games. But items drop at different rates, which creates artificial scarciry and items can also be traded for money.