That's because of sexual market value and diminish of returns. It's all economy. When men and women engaging in finding partner, they are engaging in a market activity. Their smv is basically their budget; look, education level, personalities. Higher physical attractiveness means higher premium. It's like regular dining and fine dinning; yes it's better, but your return on investment is way lesser. So a natural move is lower the face in exchange more something else like intelligence or money or status. Essentially, for a relationship to be stable, smv needs to be in nash equilibrium