The real questions here is why people given opportunity to not have children jumps at it.
My personal opinion is that children are not that great - too much responsibility, too little support from society, too many of them behaves like little psychopaths most of the time (this can be probably cured with strict discipline but stress free upbringing seems to be in vogue). And the things that children "give" in return are rather questionable (especially for people with low emotional needs like me)
Most people probably like the "idea" of children but the practical side of having them is mostly boring ungrateful work that takes greater part of you life away and there is inherent risk that it can never end (I do not understand how anyone would not take into considetation the consequnce of having special needs child)