Just like any regular computer users, hackers uses whatever OS they're comfortable with and that does the job. More likely though, hackers are familiar with multiple operating systems.
Kali's main audience is pentesters, not hacktivism. Some people calls pentesters as white hat hackers, since pentester are employed by the owner of the system they're finding security holes in. Pentesters generally don't need to worry about being discovered or "caught", since they have the permission of the system owner.
Hacktivism though, by its nature is adversarial, so it's black hat hacker or gray hat at best, because they're doing their activity for a (usually political) agenda. Black hat hackers are more concerned about covering their tracks.
It's not true though, to say that there aren't any hackers using Kali.