Immediate family, one way

I have always thought that parents and adult children were considered immediate family by the US immigration system, but now I checked and realized that this is not in fact the case, or rather, much more amazingly, it is the case only one-way: an adult US citizen can bring a foreigner parent as immediate family, but a US citizen cannot bring an adult child as immediate family. (They can, however, bring them as less-immediate family, but that involves a long wait.)

I wonder what is the reason for this? Oh course, a person has only two parents and in theory an unlimited number of children, but I suspect that in general US citizens have fewer foreigner children than foreigner parents.