Attention span is finite and Steam took a big chunk of it from gamers, in other words there is a chance that in a world where everyone hated closed platforms like steam (for not allowing reselling or any other reason) direct ads would be more favorable for developers than steam, or word of mouth, or any of it's alternatives.

Vendors give the Mafia 30% of their revenue because they know the protection racket increases their revenue by more than 30%. Doesn't that make it a good deal for vendors?