Then, there is biology: Women are attracted to powerful men, (just as lionesses are attracted to the strongest lion, and cows to the moose with the largest rack.)
Men, correspondingly, are attracted to women who look up to them. (They are not so much attracted to powerful women. Biologically, such women might be a turn-off.) In that way, the master and and the maid are often a better match than is the king and the queen.
And let's not forget physics: Men perform most effectively on women who are beneath them. That's gravity boys; mechanics that work throughout the universe.
Dominique Strauss-Kahn, Arnold Schwarzenegger, and thousands like them watch their powerful wives traipse off to fundraisers and charity events - women's conference in Gabon - and then return home full of themselves. These men can have sex with these women, but more often than not, the men will have to wait.
All the while, the maid, in her plain dress, stays quietly by the hearth - fixes his supper, sweetens his coffee, and makes his bed. She attends to his most fundamental needs. She will take the time to listen to him as long as he wants to talk. She is there.
She is a natural woman. In the master's most primal moments, she is easiest to love.