I've wondered that as well. I'm not "active" on Twitter but I log on occasionally to see if there are any interesting tweets in my feed. Every time I log on I have a new follower from penny stocks twitter, get rich quick schemes, and various other fake profiles. This seems to stay stable at around 20 fake followers as old ones get erased and new ones follow.

It seems like amateurs are more capable at detecting spam than the entire company but I sometimes wonder if they just know about it leave the spam bots because once they crack down, new ones will just pop up. Or if they keep them around at a tolerable level that doesn't drive real users away but still allows them to publish a higher "user count"

Well it's whack-a-mole isn't it? Take down one spam network and another crops up with an entirely different methodology and signature. If I was managing a large social network that suffered from bots I would whack until I came across an opponent that did the least possible damage, then weaken it through things like shadow bans etc to the point where it won't die but will operate with the bare minimum amount of damage to the network.

I think a more interesting problem is not how you can differentiate a spambot with a 'non-spam' bot. I've seen some bots that are really creative and fun on Twitter. I guess it's not really hard to add it to a spam detection ML model