Lots of people have silly, if not downright imbecilic motivations. Bikers are far from having a monopoly on that.
Yeah, it's certainly true that you get plenty of stupid from all walks of life.
But the motorcycle gang is unique. As a hobby, motorcycling does have a monopoly on territorial crime gangs. You don't see that kind of behavior associated with 4x4 offroading clubs, or bicycling clubs, pilots or boat owners or drag racers or or stamp collectors or gun owners or, well, anything else. Is there any other recreation that attracts violent organized crime the same way motorcycling does?
The closest similarity is with inner city street gangs, and that's not based on a common hobby or pastime.
It's an odd phenomenon. Why is it motorcycles, of all things?