Epic, Steam's only serious competitor currently aside from maybe GoG, just had a bug in their launcher that had all Fortnite players have to redownload their entire 150~ GB game. The cost of hosting aside, the capabilities of these companies to host their own games pales in comparison to Valve, who hasn't had a single bug in downloading or updating any game in the decade and a half I have used their launcher.

Considering how alternative storefronts can't even get automatic updates to work consistently, the most basic functionality of a games storefront (more important than purchasing even, since if you can't get what you purchased, it's useless), it actually doesn't seem obvious to me that other providers can easily host their own games. Even putting aside everything else Valve uses their cut for (hosting a community forum for every game, hosting a mod DB for every game that wants it, metrics tracking, opt-in soft DRM, providing server hosting, maintaining Proton so your game works on Linux), the cut seems almost reasonable even just for hosting when nobody else is able to do it right.