I imagine "plain women" are less fearful about the aging process.
They are more comfortable in their skin because they've never had to rely on their looks to get the things they wanted out of life.
Most likely they paid their own way through life such as focused on education, developing a career, and not expecting men to rescue them. They're not afraid to roll up their sleeves and get dirty.
It's been reported time and time again that models are some of the most insecure women in the world. They live for compliments!
Even in our era there remains an element of old thought processes.
"If a woman is (beautiful) she can get a man to take care of her."
"If a man is (successful) he can have any woman he wants."
Plain women don't have the option of being taken care of by men and poor guys rarely end up with beautiful women.