Sometimes there are reasons for children not knowing who their father is. Maybe he is married to someone else.
A one night stand. In prison or, a myriad of other reasons.

I know of a family that was schocked to learn that their father had 13 other kids besides them and 1 wife and 2 other women besides their Mom. That dog was my father, he used to say he was afraid to throw a rock in the neighborhood because he might hit one of his kids.

Your Mom may be trying to keep a family secret too. If you know where you were born the search gets easier.