Can someone explain this to me?

I never knew how hard it was for women to conceive until getting this app and becoming a part of this community and I just got the app in March.

On countdown to pregnancy it says we all have a 20% chance to conceive each month and it decreases about 5% with age. But why is that? What exactly is hindering a woman who ovulated monthly and baby dances the entire accurate fertile window, from conceiving?

If there’s an egg and a sperm present wouldn’t they automatically conjoin and form a tiny human?

I’ve tried to google this question but can never find a answer for it so please I would love to understand this better.

Thanks 🤗