Because I don't believe in it for myself. For me, that would be voluntarily making myself a cuckold, the human equivalent of avian victims of cowbirds, magpies, whydahs, and other brood parasites. Think of a house that you have to pay the mortgage on, maintain, can visit, but you can never stay the night. That's how adoption would feel for me.
I am aware that many people feel differently than I do. And, I understand well the social utility in stably-married heterosexual couples adopting.
Oh, and healthy white infants are few are far between WRT adoption in the U.S. post-Roe vs. Wade, and nearly as expensive to obtain as what we're doing. (Adopting an 8-YO black crack baby with lots of "special needs" I wouldn't consider doing at gunpoint.) And, I have some pretty good genes that I believe are a public service to pass on.