I would say plenty of people do understand what it’s like to be unattractive.

Beauty is like being tall, only some people in the world will have experience being 7’. Meanwhile pretty much everyone will have been 4’5” at some point, no matter how briefly. And if you had to be wheelchair bound one day, you’d have the experience of being “short” then too.

Beauty is not static, people’s looks change throughout their lives. The kid who plays Neville in Harry Potter comes to mind. Plenty of people get picked on while young and then become pretty later as an adult and vice versa.

I’d argue that most people are average and since it’s relatively easy to be permanently disfigured in some unfortunate accident, that it’s much more difficult to be exceptionally beautiful in the way of an underwear model, for example.

What happens though is that most people think of themselves as above average in attractiveness. Which sort of isn’t the case because most people are, by definition, average. That’s how average works. Being exceptionally beautiful reminds people that they’re just average and chips away at that cognitive dissonance. If you want proof of this, just look at the body positive movement and how hostile they can be to skinny women (especially underwear models).

Imo it’s most advantageous socially to be anywhere in the 4-8 range on a scale of 1-10. Anyone in the 1-3 range I would imagine have some sort of health condition and are truly unfortunate and the term “ugly” isn’t relevant and anyone in the 9-10 range are likely to experience the major downsides of being beautiful.