Why women who want kids can't have them?

I know this isn't true for everyone but it seems as if the women who want kids have trouble getting pregnant or are infertile. But, the women who don't want kids are getting pregnant easily. There's a lot of uneducated, people who have a good number of kids and they don't take care of them or don't want them. I know many women who would make great mothers but they can't get pregnant. It just seems unfair. Why does this happen? Has anyone else noticed this?