The problem is mismatched incentives. There's no reason for Facebook or Twitter to let you hide content that wastes your time when they profit from wasting your time (a better word for "engagement").

Remove the "engagement" revenue source (or make it unprofitable due to liability, regulation regarding privacy, etc) and most problems with centralized social media will go away (centralization isn't actually that big of an issue - plenty of things we use on a daily basis are centralized and it's not causing any issues as long as incentives are aligned).