I can make this theme very clear.
Reason 1
They(Chicks) are very insecure in spite of their looks.
Look at the BBers. A lot of them are also very insecure.
This board is full of them.
So many(Not all) times taking care of outside means inside is very insecure.
So thats why they choose guys with bad looks for their security(avoid his boyfriend having sex with another or criticism by her self or third person).
I am not guessing this. I asked many good looking women about this phenomenon.
Reason 2
The guy respect the ego of that good looking chick a lot from bottom of his heart.
Type like artist or romantist.
Reason 3
Money
I personaly opined that 99% of this phenomenon can be explained with the brend of Reason 1 and 2.