For the same reason that the mother committed the Fraud in the first place. THE REAL FATHER HAS NO MONEY.
It's like the old joke - "why to crooks rob banks? Because that is where the money is!"
Many women, bless them, have a thing for low-life bad boys who are usually good in the sack, but not good husband or parent material. These women usually have multiple concurrent sexual partners anyway, and they simply choose the man with the most cash to list on the Birth Certificate as father.
They could care less about the parental rights of the NATURAL father, they only care to get paid.