Because the rest of us have a right to life, too. Ever heard the saying, "Your right to swing your fist ends where my nose begins"? That's colorful, but it's not even true; people have an expectation of a certain reasonable amount of space around their bodies, and even entering it with your fist might be considered assault. The concept that one's actions and choices affect other people is what's important here.

That's the problem with giant pickup trucks: They affect other people on the road, and the problem with giant pickup truck drivers is that they either refuse to recognize this fact, or they enjoy infringing on the rights of other people to enjoy life. Either way, it's bad for society, where we all have to live together somehow. Mullets and man-buns, by contrast, don't materially affect anybody else in the slightest.