It has been discussed. The problem with using an actual currency is actually mainly gambling. A lot of mini games and stuff in these games are RNG based. If you’re using real money, it’s no different than slots. Equating your ingame currency to real life currency just becomes all sorts of legal headaches that gaming companies do not want to deal with.

Crypto is essentially digital currency. Games like RuneScape have had digital currency for sale on secondary markets for over 2 decades now. Same with wow, diamonds in mind craft, or any other in game tradable currency that can be sold for real life cash or other assets like BTC.