Why is it thought that a father only wants custody because he doesn't want to pay child support?

There are dead beat dads and moms,he was ordered to pay because she got custody.There are alot of men now raising children without the moms in the picture and laws are starting to change when before it was the mom who always got the kids even if she was a poor parent.When my parents divorce women were only allowed custody of 3 kids so there fore my brother and i were raised by my dad.