A core feature of anti-fatness is the "you did this to yourself"-view. People are very invested in the idea that size is within everyone's control. It's soothing to believe that all fat people are a small series of good choices away from becoming thin and staying that way, and that thin people are success stories by virtue of existing.
Any time we speak up about discrimination and fatphobia, someone inevitably plays that card. Trolls will say "eat a salad, pig" and well-meaning health nuts will gently explain what calories are. In either case, we're met with a "you know, you can stop this at any time." Why, if nobody was fat, thin people wouldn't need to examine their biases! It sure would be an easier time for everyone if we weren't so Around and Bulliable!