Can someone PLEASE explain to me!?!?

Okay, I'm so confused as of why everyone always says "God will give you a baby when it's time" or whatever people say, because I have a 14 yr old sister that was on drugs and was waaaay too young to even think about having sex get pregnant and she had a healthy beautiful baby boy. Yes he is fine and well taken care of because of our parents.

Theirs just too many people that obviously shouldn't be having kids, and people that dojt need kids.

Here I am happily married and stable to enough to have a baby but me and my husband have been trying for over a year and it hasn't happened yet. I just don't understand!