Because salad is boring and fat-shaming is the last kind of bullying still considered acceptable.
I was out on my bike the other day and someone yelled "YOU FAT BASTARD". Fortunately I'm pretty thick skinned and have lined up a few choice remarks for next time.
Imagine if fat had been replaced with black, or Chinese, or gay, etc. They'd be in jail for committing a hate crime quicker than I could get to the nearest Greggs.