What a relay wants to store and what a client wants to retrieve are not well aligned. A relay just wants to avoid cache misses. But a client wants certain infrequently accessed data to always available. This make things like profile data grow stale and get removed even though it is still logically pertainate while a note with the same access pattern is not.

It also isn't as simple as just holding all the kind 0 information forever. If you do fiatjaf will make a bot to spool up tons of kind 0 records.