Likely because if bad actors add bots and gain followers (across all bot accounts) faster than Twitter kills off the bots, the ratio of bots to real people will

Likely because if bad actors add bots and gain followers (across all bot accounts) faster than Twitter kills off the bots, the ratio of bots to real people will eventually (on a long enough timeline) become infinite. When people notice they're only seeing, hearing from, and seeing retweets from bots, they're likely to leave Twitter. That would be bad for Twitter.