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