The network effect of having established a marketplace with millions of consumers who pay for games is non-trivial. Steam is a trusted name in gaming for a reason.

Steam also promotes games, has reviews of them, and suggests games directly to players. The first time I personally heard of many of the games I play has been through Steam, Humble, GOG, Epic's game launcher/store app or other launchers and game selling sites.

I also can reinstall hundreds of games from Steam without remembering who developed it, who paid for the development, or who published it to Steam. I just tell the Steam client to install it based on the name of the game. That's a great usability and convenience feature.