The answer is because the law makes adopting children in the US difficult.
Few babies available.
No cross racial adoption.
Lesbian social workers doing your home evaluation might turn you down for being a Christian, saying you are too “ rigid”.
Older children often have emotional or sexual abuse. And since social workers keep sending them back to their druggie moms over and over and then remove them when mom relapses, and they bounce from foster home to foster home, they often have behavioral problems and can't form attachment to you.
The behavior can be terrible, but adoprive parents mainly don't discuss it with strangers, and luckily most kids outgrow their acting out and do well.
True, adopting from overseas doesn't eliminate such problems, but adopting a young child means probably fewer of these problems...and adopting a younger child from overseas is easier than adopting one from the US folks go overseas.
To adopt requires a home study. True, private adoptions might not. But adopting from overseas does.
Some get around this by paying a surrogate mother in a poor country to have the man's baby, and then he gets custody.
Informal adoption is common in some areas...including neglected children who are taken in by neighbors or relatives informally, without legal intervention. This includes family caring for a teenager who is pregnant and raising her and her baby until she is old enough to make it on her own.
This doesn't include the one million children in the US who are raised completely by relatives, often grandparents, because mom was too young, immature, or nowadays alas because she is a druggie.
Well, Ok; good reasons.