Developers aren’t forced to exclusively ship on Steam or not at all.

That’s just not true in practical terms. If you want your game to be discovered and you don’t have a massive advertising budget, it’s not a serious option to try to forego selling on Steam while staying in business as a game developer. That’s like saying Amazon isn’t an ecommerce monopoly because you’re not “forced” to sell there, even though that would mean bankruptcy and irrelevance for most sellers.