It Is Because of Sexism

That is the reason there is an obsession with thinness in our culture, and almost all of it is directed at women.

"Obesity" has very little to do with consumption of food and everything to do with hormonal changes that happen to women as they approach or exceed 50. Thyroids get shot to shit, lack of estrogen forces the fat to migrate from the hips and thighs to the waist, regardless of how active a woman is. But women are relentlessly hit with this thinness crap because women who are "heavier" are not attractive to (most) men, regardless of how shitty most men look. Once women hit menopause, they basically become invisible or unfuckable to men (unless they are already married or in a shackup situation).

Rather than going through life without the "love" of a man, women will do everything in their power to remain "fuckable." Women will ruin their health to meet some kind of arbitrary "beauty" standard. In the end, though, age happens, and at some point women are no longer considered "attractive" enough to get a man. This happens as early as age 35 or 40, but definitely happens by age 50.

Weight has little to do with health, the medical profession's collusion with the diet and fashion industries notwithstanding.