How come there are a lot of white couples who adopt black children ... but never the opposite?
I know a family who has a white adopted child who are not white themselves, so stating it never happens is not accurate. In general, white people adopt more often than black people do, and there are more white foster parents than black foster parents as well as fewer white foster children than black foster children, so it is not at all uncommon for a foster family who has grown to love their foster children to adopt them if the children become available for adoption. There are more white families than black families who do international adoption as well.
1. There's more black or other minority children available for adoption than there is white children. 2. There's more white parents seeking to adopt than there is other races. To answer this race bait question again, black people can and do adopt white children and otherwise children outside their race as well as black children. White people can do the same.
"Never" is an overstatement, since black couples do foster/adopt outside their race, too. It's more likely for white couples to foster/adopt black kids simply because there are more black and other POC children in need of loving homes. Healthy white female infants are most likely to be adopted (because they're most in demand), while black teen boys with special needs are least likely to be adopted.
Adoption rates would be a product of stable, two-parent household rates. Blacks have a much lower marriage rate, higher age of first marriage, higher relationship instability rates, higher single-parent household rates, and higher poverty rates. All of those factors would translate to far fewer capable, stable, married black adoptive parents. It's not that they wouldn't like to adopt, but few have the ability.