Why are some woman NOT ABLE to conceive? Is it because they have UNHEALTHY LIFESTYLES or LEAVING IT LATE?

Forgive me for asking. I'm a 28yr old mother of 4 kiddies. To be honest, my husband and I only have to look at each other to get pregnant, I'm actually about to pop any day soon so this bamboozles me. How come some woman find it so hard? do you think they are just not a reproductive match to their partners or perhaps...dare i say it....is it nature weeding out those who are meant to be mothers from those whos bodies are not able to create a baby? Doesnt that mean that these people are best leaving it to nature besides pumping themselves with artificial drugs? I mean, lets face it, IVF is just a money spinning machine and if a womans body doesnt work, it doesnt work? Just asking....