Most Helpful Guy
Because people don't want to be judged automatically and called shallow. Looks are just as important in my opinion. YOU have to be attracted to them, doesn't matter what anyone else thinks.1
Most Helpful Girl
It isn't a lie. I have had crushes and have been attracted to guys who would be deemed downright ugly by most people due to the traditional notions of "attractiveness" and "good looks". Some people need good looks in a partner. Other don't.
Also, this is especially true for people with average good looks. I consider myself average. My boyfriend said the same thing to me way before we were friends, that the brain matters not looks. I am overweight and short. But he has eyes for no one else.
It's possible that lesser people think like this though. I don't think women say this much because it's too true for them. I think men say it more because there's this notion that men only want the chicks on magazines or porn. So they say it as a disclaimer to make you know that they are not one of those guys.
Also, it's a way of making you focus on the aspects of your personality that are more advantageous than your looks. Another thing is that they would ALWAYS prefer good looks+good personality, so if you get that, then you're better. But what they're trying to say here is that if they're not getting that combo, what they prefer is average looks+good personality over good looks+average personality.0