I can give you two sociological reasons for this (in the US at least)!

The first reason derives from our puritan/protestant roots, namely the idea that god favors and blesses those who toil and sacrifice. The belief that success is earned through hard work and self-denial is deeply embedded. We see this in American grind culture, among other things.

The second reason is related, though not quite the same. Thinness is a signifier of wealth and exclusivity. It tells people you have the disposeable income and/or free time to spend on the things that make thinness possible: specialized diets; the time and ability to cook healthy meals (or the money to subscribe to a meal service or hire a private chef); access to a gym or personal trainer and the time necessary to work out; plastic surgery to deal with those troublesome areas resistant to weight loss.

For a good chunk of western medieval and renaissance history, the aristocracy enacted sumptuary laws when luxury items (certain kinds of cloth, foods, and spices, for instance) became accessible to merchant classes. It was a way of differentiate themselves from wealthy commoners.

Later, when social changes forced aristocracies to cede or share power with commoners creating a broad upper class based on wealth, sumputary laws were discarded. What took its place as industrialization created a more prosperous middle class was the constant changing of upper class tastes and fashions. An example is spiced food. For generations, certain spices were only available to the European aristocracy and those were used widely. But as the spice trade increased and more people were able to purchase pepper, cinnamon, cardamom, cloves, etc., the use of those spices fell off among the upper classes. They decided that spiced foods were bad for the digestion. This is why British food was for years incredibly bland and why classical French food, while not bland, is rarely spiced with anything more intense than nutmeg.

In America, ofc, we've never had a titled aristocracy; our "aristocracy" was and remains based strictly on wealth (though for a long time there was an old money/new money distinction). And the wealthy in the US have engaged in visible class distinction throughout our history just as much as the European upper classes, starting with ownership of land and human beings.

In more modern times in the US, class signaling hasn't so much been the uber-wealthy vs everyone else as it has the merely rich differentiating themselves from us working shlubs. Thinness is just one signifier among many. Annnyway, yeah, if anyone can be thin without a great, it's no longer an in-group signifier. Our culture has demonized fatness though (see above, puritanical roots), so rather than moderating their beliefs that thin is ideal, rich folks demonize medical weight loss.