Because they can and there are is no real competition. Epic Game Store so far is laughtable attempt at pumping money into user-hostile service that missing tons of features. For now the only good reason to release on EGS is Epic giving your company money.

Disclosure: I work at small indie PC gamedev studio.

I just point out how Valve changed rules for big companies back in 2018. So they have different revenue share compared to both Apple / Google and consoles.