In the UK this discussion has been prevalent recently. Interestingly here, to adopt a child from a different country requires you to go through the UK adoption process first, and then the process in the country from which you want to adopt. The main difference, however, is that in these other countries it is possible to adopt a brand new baby, whereas in the UK the baby is almost always a year old by the time they are given to the adopting family. That is a significant reason for people in the UK adopting from elsewhere.
The other most notable reason is that the conditions babies are having to live in in some of these other countries are so dire, that people feel compelled to adopt these babies in most need. There is just no comparison to how well a baby needing adopting in this country will be cared for, compared to those born in certain countries and left with penniless orphanages.
I imagine this latter reasoning, at least, to be a key part of answering your question on American babies.