Why not is a good question. The answer is the same as the one to why not pick on people injured in car accidents or while participating in sports or military combat. Those were all choices too, and some might even say immoral ones.

When someone picks on fat people, it’s because their arbitrary moral code is against gluttony, but apparently pro anger and pride, which are also deadly sins according to Christianity. It is obvious that their judgment is arbitrary. So what it comes down to is status, and the person who perceives themselves as being of a higher status, attempting to lord over someone they perceive as having lower status.