why is Conceiving at best only 30% chance?

What is the technical reason why conceiving is only 30% chance, even for the most fertile women? I'm 29, have tracked my ovulation/ periods a year, just started TTC first month and just got my period today. My partner and I were very sexually active this past month, especially during my fertility window. He has two beautiful children from past marriage, and I sadly under unfortunate circumstances had an absorption about two years ago. So, both of us are capable. I know even up to a year trying is totally normal for my age, but curious about why it can take that long. Thanks :)