Because rape is a thing.
People not being able to afford. Before some asshat says just dont have a baby if you cant afford, contraception isnt 100%, so alwazs that chance and we arent going to honestly monitor all the folks that cant afford a baby to make sure they arent having sex/neutering them all.
Complications, birth defects, etc as well.
There are even more reasons, but these are the easy 3.