Because men have to get married if there's going to be a responsible marriage-and-family culture.
Why would they?
Your solution (lots, lots, lots more Norplant and IUD's) only contributes to the further disintegration and decay of anything even vaguely resembling a human culture --- as opposed to what you'll find in the oozy bottom of a never-cleaned garbage can.
Subsidizing the broken families (and those who are on welfare are almost never married) does not help society.
Ending the subsidies for more fatherless children will incentivize proper family formation, even if it simply means curtailing the growth of a socially broken underclass so that married women have a lower financial burden and can afford more children.