I suspect it's because the Twitter bots inflate the user count of Twitter. It seems that even though Twitter's monetization is not the greatest, their valuation mostly comes from the number of users on Twitter. The reasoning process seems to be something like, "Twitter doesn't make a lot of money right now, but they have the ear of some 300 million users, and that's going to be valuable in the future when they get their monetization story correct, right?" And so TWTR prices are kept afloat. Bots probably degrade the user experience of Twitter quite a bit, but if Twitter aggressively bans all bots, their user count goes down maybe 15%[1], and what does that do to their stock price?
[1] https://arxiv.org/pdf/1703.03107.pdf