It's just like saying why are all girls rude to guys? Not all guys are mean and disrespectful and not all girls are mean and rude. But the guys and girls who are mean, rude, and disrespectful are just jerks.
Boys are stupid! hehe jk I think it's there way of trying to seem cool and masculine. but don't take anything they say to heart. because if you ask them what they said ten minutes later, they'll probably have forgotten(;
In order to be respected, you must expect, and demand, respect. The boys who are disrespectful of girls are because the girls they know tolerate their disrespect. So, really, in a sense, the boys are disrespectful because (some) girls taught them to be. Stop and think: before the Women's Liberation Movement, women had to put up with a lot of discriminatory behavior from employers, from doctors, from men in general. But after women started demanding their rights, those things began to change. The same is true of the respect thing. So, if you want to be respected, woman up and demand it!