People idealize having children. They think it will make their marriage stronger. They pump them out faster then their incomes can provide for them. Mostly, people are stupid, and selfish. This is the short answer.
Some of these folks blindly believe in what a certain religion teaches them about the "evils" of birth control, yet that same religion can't reconcile those rules to their own book (bible). Lemmings abound. People allow others to do their thinking for them. They don't check what they've been told. They say "we are traditional".
If they only knew, that their God would NOT approve...
Too often 50 something people are struggling, having to live with the consequences of a stupid 20 something year old...
Yup! And remember, their religion is based on an accidental child born as a result of the lack of planning and likely infidelity. I mean, either Joseph was pretty gullible or they made up that story to cover up their accident! Once you start believing that then believing that god will always provide, any stupid behavior becomes easy and a good excuse for lack of accountability.