Because body positivity is fundamentally and wholly linked to beauty, which is an unworthy candidate for our time and attention.
It occurs to me that beauty HAS been a part of our time and attention since the beginning of recorded history. I think we can assume that that’s not going to change anytime soon.
But mostly, and this is the big one, it doesn’t matter if a woman is beautiful or not.
This is of course true. There are a lot of avocations that “don’t matter”, but that we (speaking of both sexes now) quite enjoy. And we enjoy fussing over ourselves, for whatever reason. Men prefer sports, I suppose.
Perhaps your central thesis ought to be, instead of a strident “stop caring about the way you look”, that beauty needs to be relegated to the status of a hobby rather than an obligation.
It occurs to me that beauty (and fashion) is a bit like doughnuts; they’re only a problem when you overindulge. If you agonize over your looks, and you keep trying to “rate yourself” based on some comparison to a twenty-year old model who has had “work done” and appears when photoshopped to within an inch of her life, then your beauty hobby has become an unhealthy obsession. Seek help.
That said, there’s nothing wrong with wanting to look your best, even if that “best” only appears after a good bit of fussing over yourself.