Evo psych has provided explanations as to why certain features are seen as attractive. It mostly has to do with ability to survive in primitive conditions. For example, tall and muscular men are attractive because they can be better hunters, protectors etc. Women who have poor skin and hair are percieved as ugly because it signals poor health. People who are fat are less mobile and agile, also more proned to cardiovascular problems. Since you don't want to pass those traits to your offspring and therefore reduce their chances of survival and procreation, people who have large % of body fat, bad skin, hair or low muscle mass are instinctually percieved as sexually unattractive.

But isn't there a discrepancy between the modern industrial way of life and those instincts? For example, even though mucular people could be better hunters, you don't obtain food by hunting anyways, you just go to the store and buy it. Even a short, fat and ugly person can go to the store to buy food just as well. And maintaing good health is way more dependant on having access to health insurance than having physical traits like nice looking hair or skin.

Conclusion: People have instincts that evolved in time when the context of life was different. They are attracted to physical traits which act as signals to ability to survive and reproduce, but are contextually pointless today

Theory: Women are better adapted to modern times than men - tall and muscular guy with a low paying dead end job is less attractive than a guy with average physique who makes 500k$ a year. Men - prettier woman with smoother skin with low income and no health insurance more attractive than an uglier one with bad skin and a good job/better living standard, even though the latter is logicaly a more sound choice.