They go wherever the people are at, so they can train, inflate user population, and influence opinions.
It costs them almost nothing to be at multiple places, at the end of the day it’s all text to be parsed and people to manipulate.
Actually, usually when I see someone questioning if bots exist I think of that as an actual bot trying influence people away from thinking about bots, considering that bots are all over the place at this point, it’s weird to see someone deny/question that.