GOG is definitely superior as far as respecting the rights of consumers is concerned, but their web store and library client (Galaxy) doesn't offer nearly as many features as Steam. Things like hosting mods and making them super easy to download/install through Steam Workshop. Or things like streaming gameplay to a different device (or even streaming your normal desktop allowing you to use it from a different device so you aren't just limited to Steam games). Or WINE emulation on Linux with minimal configuration needed. Steam provides some legitimate value-add features that pretty much no one else does, making it much more than just a storefront.