Walled gardens exist because it costs money to keep the garden flourishing.

Keep in mind, in this walled garden, all the walls do is make you log in, everything else is still free.

Decentralization still requires someone to pay for keeping these services alive. You could argue every user somehow be able to own and run their own platform, but in reality most of the users will not opt for the overhead when you can get it for "free" from a provider. And in a semi decentralized world where a few separate providers keep the lights on, the providers would still need to be compensated for the time and resources they invest, which again brings you back to a different kind of a walled garden.