Steve Bate has a long blog post about why ActivityPub’s client-to-server protocol has not been implemented:
Yet despite its promise, ActivityPub C2S has seen minimal real-world adoption. Most Fediverse platforms — including Mastodon, the dominant implementation — have actively avoided supporting it. Instead, they expose custom APIs that tightly couple client behavior to server internals.
More clients should use Micropub. It is also a W3C recommendation, but unlike C2S, Micropub is already widely implemented and has evolved through extensions to accommodate most real-world use cases.