Why is it that men are not considered custodial parents to thier kid ?

The system is out of order ! I am a farther and I find it rude and not fair for just the mothers to be looked at as the custodial parent only. Why is it that as a farther I am put in the garbage; I have to sign for rights or set in a motion to file for custody to obtain any right; While the mother can move out of state and not say a thing to the farther.