The more simple approaches have already been tried and tend to die before they live.

Social media requires a network effect in order to be successful. Given the established players have had nearly 2 decades to accumulate vast networks, it would be a huge uphill struggle to start from zero content and users. Federated & decentralised social media is the answer to this—you get a network for free, giving the software a chance to stand on its own merits.

For this to all work correctly, they must all talk the same, ideally standard, language (the activitypub protocol) and for decentralised software to actually be decentralised, there can be no single point of failure (therefore caching). As someone mentioned, SSO is inherently centralised, even with something like OpenID, if your authority is down, your account is unusable, so it wouldn’t really add much to the experience as it stands (and possibly may risk complicating it more for new users).