Because developers go where the users are. That's all that matters.

Epic had to pay developers, a lot, to bootstrap the Epic store so that there were games there that mattered. And even now, with a decent selection, most PC gamers still use Steam. Because all their games are on Steam already. And every game they care about is on Steam.

So Steam can charge 30% and Epic can charge 12% and nothing really changes.

Probably has to do with market power. For my money, none of these companies should be allowed to own a game marketplace. The distribution channels should be stand alone from the hardware and software producers.