And because things must be strongly consistent, AND because any user can REWRITE THEIR OWN FUCKING HISTORY AT ANY TIME, you have to, as an indexer, account for lots of different edge cases where your recorded history diverges.
The protocol for federation is built to make federation as difficult and painful as possible. It is built so that Bluesky, the private company that makes the protocol, is the only 'indexer', the only one with a whole view of the network.