That's true, but if the NFTs show up on some places and not on others then you could start a "resistance" against the existing market places. Outside of DMCAs and other such legal requirements, an exchange needs to be impartial about the stuff being sold and published on there to remain credible.
The cryptocurrency crowd is usually drawn to the decentralised, unregulated market, and OpenSea has turned out to be the exact thing blockchains are trying to overthrow.
But that's the point. So many people hyping web3 like it's going to be fundamentally different which is why it's worth all these resources (both people and energy), when it appears already heading down the same path as web2. Consolidated companies growing very large and getting a bunch of already known VCs even richer.
It's like a populist movement whose goal is to enrich the existing rich.
The cryptocurrency crowd is usually drawn to the decentralised, unregulated market, and OpenSea has turned out to be the exact thing blockchains are trying to overthrow.