The subscriber tag has additional data that’s impossible to obtain through the API, such as what sub streak range they are in. Also subscriber badges, and various tiers of those, was a HEAVILY requested feature by streamers, so the sub tag is a key feature for chat functionality, the same can not be said for a follower tag.
The bots I’ve seen that commonly use a ‘regulars’ setting for commands don’t rely entirely on whether a user is a follower or not, as someone who just joined the channel and tapped follow (or even was already a follower due to the mobile apps onboarding) is not a ‘regular’ viewer. To use anything like view time, or number of streams watched, activity in chat, or any other metric to determine if a viewer is a ‘regular’ is in no way helped by a follower tag unless you’re also asking for way more additional data being added too.