…the natural parents of adopted children don’t impose adoption on them on purpose…
Surely this isn’t universally true. Some natural parents get pregnant, consider the options (raising the child themselves, abortion, or adoption), and choose adoption as the choice that makes the most sense for them (or for her, if the father isn’t part of the decision). That’s “imposing” adoption on children on purpose.
My guess is that this reason for adoption is, if anything, more common than those parents who leave their children to be adopted because they die in a car accident, or some other truly involuntary event.