> There's literally a popular decentralized social network.

No there isn't. Not a single one.

There are a few federated social networks, which is a fancy way of saying that they are centralized networks that have (or can have, in principle) more than one "center".

In practice, the overwhelming majority of users of such networks gravitate towards one or a handful of large providers. And many of those providers actually refuse to federate with other providers unless they follow an ever-growing list of politically-charged rules. This is just centralization with extra steps.