My guess is that the secret sauce is network effect. People have all their games in their steam libraries, so they want the next game in their steam library too.

They perceive the Steam launcher as something positive that provides them convenience. The network effect also makes it hard for game companies not not be on Steam, allowing Valve to enforce some customer-friendly policies like refunds, further making the platform nice to use.

Epic is trying to break through this by giving away endless free games. All other stores are generally forced upon the player through exclusives (you buy the game on Steam, but you still have to install their shitty launcher-store because the parent company owning the game wants some of the store-platform cake), breeding resentment and further cementing Steam's position.

(GOG being an exception, but they don't have enough major/interesting games to get players to use them as their primary platform.)