They can take a cut of every secondary market transaction and players need to be able to cash out for a secondary market to make sense. The main reason for not doing that centrally is it becomes an attractive means of money laundering which Valve found out with their skins. Second Life also had this problem and IIRC a third party company handles all their game related banking business as a result.

Note I’m not saying this is a good thing and I think all the entities involved should have the same regulatory responsibility but you can see the greasy cogs whirring in some executives brain.