To play devil's advocate briefly, assume that Twitter decided to delete all accounts it flagged as spam/bot accounts tomorrow. What would the long term fallout be? It's unlikely that people selling follower counts would simply go away, just become a more active adversary. By putting bots simply behind a quality filter (a pseudo shadow-ban), they avoid an escalating arms race in bot account creation.

The real fix here is to stop showing follower counts completely, since they're trash.