In theory, being able to follow arbitrary object could be useful in certain cases, but that blurs the line between actors and objects, and in my opinion, creates more problems than it solves. Furthermore, ActivityPub doesn’t doesn’t support this use case. According to section 6.5 Follow Activity, only actors can be followed:
The Follow activity is used to subscribe to the activities of another actor.
Again, ActivityPub (overview):
In ActivityPub, a user is represented by “actors” via the user’s accounts on servers. User’s accounts on different servers correspond to different actors.
Mastodon might be not right about hard-coding the list of actor types, but mapping users to actors is literally what specification tells us to do.