Flamethrowers aren't precise. Say you go out to kill a bunch of politicians but in process you might as well kill insects, small animals, birds and even vegetations. And we only wanna kill humans. So semi auto assault rifles are best. Precise.
Doesn't fit the pesticide category tho. Also pesticides are not always precise, you might go out to kill weeds and end up killing families of ants, flowers and maybe some jews.