It's very simple - anyone can assert anything, thus each individual must take it upon themselves to be skeptical of the assertion, or blindly agree with it.
The real problem you're getting at here is that many people are dumb, and may be made to believe anything.
If these people became skeptical or were simply deleted from reality, such bots would have no value.
Was this Russia?
https://www.washingtonpost.com/archive/opinions/2003/02/06/i...
https://www.washingtonpost.com/technology/2018/11/15/faceboo...
Verified by multiple sources. "Facebook said it had found and removed roughly 1.5 billion fake accounts" surely doesn't suggest they're all literally Russian, or bots, but they sure were fake and spreading propaganda.