Well, from a consumer's prospective, there's no compelling reason to spread out one's purchases (if the option exists to get them all in one place.) Having to download a separate launcher for each game isn't necessarily a deal breaker, but it's certainly not convenient either.
Imagine having to manage all your phone's apps through different stores, launchers, and website downloads. Sure, you can. But it's not as seamless as having all your apps in one place.
If a game is 15% cheaper or has a larger discount in a different store then why not buy from that one, and if I have to choose I will buy from GOG because of DRM free, if that is not possible and prices would be equal on store A and B I would buy from the store that gets more money to the developers because I want them to get enough money to continue the games.
Imagine having to manage all your phone's apps through different stores, launchers, and website downloads. Sure, you can. But it's not as seamless as having all your apps in one place.