I think part of the problem is that there are people who eat heathily, care just as much about their bodies, and do lots of exercise, who still aren't like the ideal of physical appearance and weight promoted all around us. Some of us might have sections of our bodies that we feel embarrassed about which even in other cultures might not be seen as horrible and somehow our fault, but actually as attractive. The body-positivity movement isn't really about 'it's ok to stuff yourself/do no exercise/endanger your health through bad eating and no exercise'. Because even if we all ate and did exactly the same, there would still be different body shapes and sizes. So it's about if genetically for example you have more curves (I don't mean euphemistically 'fat', I mean shapely hips etc) or bigger boobs or wider thighs, you shouldn't feel ashamed because you don't have stick thin arms and legs and a concave stomach. It applies to young people, maybe girls in particular, who just have a different shape, to women as they go through all these stages of life with their bodies. To men who don't have muscles and do have a bit of a dad bod. etc.
If you don't look on social media, where are the examples of media promoting obesity? On social media I'd say there's a drive, a very good one imo, for everyone to absolutely make the best of themselves and cherish their bodies and prioritise self-care, and not be perfectionist. A positivity movement usually comes out of a reaction to some kind of crushing prejudice. I don't want to feel that I should be deeply ashamed of my tummy after childbirth and that there is a hierarchy - those who look 'perfect' feeling somehow superior to those who aren't. In some places there is way more body positivity and it's interesting because in those places, the women look a lot happier, there's more variety of body shape, and people have smiles on their faces. I've moved from a place up North where even the oldest ladies in the gym changing rooms were putting on bright clothes and lipstick and looking great. Now I'm down in the South East in a supposedly desirable area where the women might all be uniformly slimmer, but omg do they look miserable/mean and grey. Obviously not everyone but ... and I know that if 30% of those ladies from up north were to walk down the street here, there would be looks of shock and horror on the judgemental faces.
It's this kind of thing that body positivity tries to address. To redress this unbalanced and unpleasant attitude.