Why does it take so long to get pregnant?

This might be a silly question but I figure insight from all you lovely ladies out there would be a good place to ask. If you are TTC and you have sex every day when you are ovulating one or more times, how do you not get pregnant (aside from health issues)? If you are having unprotected sex multiple times and you are ovulating what is happening in your body that prevents the egg from being fertilized? I am trying to understand how it takes fertile people years to conceive. If this does offend anyone I am not trying to in any way I'm just seeking knowledge! Any insights would be great