That is mostly because there are very few servers that support the client to server portion of the ActivityPub specification.

I am working on such a server, and I have built a custom client for it, but it's just a web application.

... what does that mean?

It sounds like you are saying "there are no standalone SMTP clients", after which we would all start listing software that speaks SMTP.

So point out the clients that speak ActivityPub?

There is a lot of software that speaks the ActivityPub server-server protocol, but most client apps use the Mastodon client API or custom protocols, not ActivityPub client-server.

SMTP is strictly a server-to-server protocol. POP3 and IMAP are the protocols for accessing mailboxes remotely.

You can send mail via smtp connection from client to server

Woopsie, totally forgot about that part. It seems like I got too used to webmails.

Likely because ActivityPub alone provides just a subset of what a typical social media client needs, and requires far more logic in the client than using the Mastodon API. E.g. if you use those APIs you don't need to care about webfinger and can treat the local server as the only API endpoint. There are few compelling reason for client writers to talk ActivityPub to the servers unless/until there's a far larger number of servers which does not support the Mastodon API

AndStatus definitely speaks it