Of course babies do not come out of the womb ready to take on the responsibility of maintaining and improving our infrastructure and productivity. These babies, though, have a much higher return on investment, since they will still have their entire adult lives ahead of them after the country educates them. They, as children, are also much easier to educate and have built in language and culture teachers in the form of their parents.
Immigrants, on the other hand, will want to start working and earning money immediately, and will probably not wait through several years of language, cultural, and professional education.
The idea is to establish and maintain a steady stream of educated workers into the population, and the easiest way to do this is to encourage the present population to have children at the replacement rate.
World adoption is part of the solution, and many Americans do adopt children from other countries. These adoptions are by people who are willing to have a child in the first place. The government could encourage people to adopt rather than give birth, but giving birth is cheaper, faster, and easier. The people who are willing to adopt are usually just as willing to give birth, but can't for whatever reason.