Why do people care about diet/looks so much?

I’ll always get angry at fatphobia and lookism because I believe in body autonomy. Like why are we constantly hating on people for being fat. As someone who’s currently fat, when I was skinny I always wondered why people pointed at the kid who super fat when they were just existing. As someone who also wears heavy makeup I overheard people talking about my face with negative comments. I just don’t understand why we live in society that cares what other people do with their bodies. There’s been times when I’ll argue against fatphobia and some would bring up unhealthy diets. In my mind, okay if someone is unhealthy with Their diet who are you to hate on them.