There is a federated system of deletion, it just doesn’t have guarantees to work. The problem is that it’s a distributed system among untrusted actors. Usually the only way to have a distributed, secure, untrusted system is to use a blockchain. In the absence of a blockchain or equivalent, we keep the “distributed” and “untrusted” but concede the “secure”.

This doesn’t just go for deletions. It also means submissions or edits might not affect other servers. Malicious servers could also change all your posts to say “I’m an ugly moron” and those could get propagated to other servers.