Hugh on Nostr: As far as I understand, most (all?) fediverse #ActivityPub software does not use the ...

As far as I understand, most (all?) fediverse #ActivityPub software does not use the Client-to-server protocol from the specs (https://www.w3.org/TR/activitypub/#client-to-server-interactions) but rather use custom APIs instead.

Any fediverse devs able to explain why? Is there a technical reason/limitation, or is it more about other considerations?

I'm looking for information here rather than speculation, thanks.