Because it doesn't work. Back in my day when Cindy Crawford was the gold standard that was at least attainable for me. Once Kate Moss and the waifs entered the picture things got a lot harder. That was fainting, dizzy, hunger pains all the time, bruised butt territory and I couldn't sustain it for very long. I'm not big boned, but even at "too thin" I'm still a size 8 and that just wasn't good enough in my size 0 or nothing 20 year old head. I don't think thin women are ever going to aspire to look like a size 14 model, but it might inspire a size 22 woman to get to a healthier weight, even if not the healthiest.
When I see people, mostly guys, cruelly fat shaming all over the net, especially YouTube, it is not inspiring. It's defeating. Yes, we all should just want to be healthy, but for women especially that can fall within some pretty wide variances and then there is healthier, healthy, and healthiest. I believe some older studies have shown that slightly underweight is actually healthiest, but I don't think we're helping moderately to severely obese women to lose weight by name calling "chubby" models, celebrities, and you tubers.
Shaming men for preferring thin women is also wrong. Our sexual preferences are largely out of our control and men have been getting the same size 0 ideal messages for however many decades now too. When you feel good about yourself you want to take care of yourself. Sometimes feeling like you can dress well while overweight can actually be a positive motivator to lose weight. I know I lose weight with the least difficulty when I'm also consciously trying to look my best at the weight I'm at. Success begets success. Derision and contempt are more likely to inspire throwing in the towel and giving up.
Additionally, I find what I perceive as a strange assumption that any woman who dares speak up should have to pass a beauty contest and weight limit if they want to be worth listening too very frustrating. It's especially frustrating since outside of the realm of dating and romance one has nothing to do with the other. It's not that black and white of course as even men in business are seen as more capable leaders when they aren't obese, but it does seem that in general men can get away with more extra pounds before they become virtually invisible. Chris Christie has shown us that fat men get shamed as well and I can certainly admit that that much girth does not inspire confidence.