Why is it difficult for some people to get pregnant?

I don’t mean to come across as too naive but this topic sincerely bothers me. I have always thought ever since I was a young teen, that the first time a woman has sex, she gets pregnant. Then as I got older I realized that it can more likely happen only if the man ejaculates inside. Now that I’m in my 20’s and completely new to this whole pregnancy thing, I had just learned about ttc because of this app and other apps. It concerns me that a lot of women say that they “struggle” to get pregnant. Why is that? I just don’t understand why for some people that have sex for the first or few times without trying for a baby can get pregnant successfully and other women struggle with getting pregnant even after trying for over a year. Someone please enlighten me on this! x

Also please don’t be rude because i’m just genuinely curious 💖