Because women NEVER lie. And of course they NEVER cheat.
If he was married he would probably get hamstrung for child support even though it's not even his child. Is this a great country or what.
In most states, if his name is on the birth certificate he only has a certain amount of time to challenge paternity and get his name removed, usually about a year. After that point, he is on the hook for support, even though he can prove the child is not his. Just having the DNA test in hand is not enough, he has to get a court to remove him as parent.
A very tough lesson I’ve never really learned. I was raised to believe that women were honest, righteous, and generally above reproach. Not very realistic, but that early conditioning isn’t easy to shake.