The US is the last hold out of a modern country that still grants citizenship to anyone born on our soil. This had it's place decades/centuries ago but not today when we can fly anywhere in the world in a day.
This is also a lure for illegal immigration. Expectant mothers put themselves and their unborn children in danger to try to get into our country to have an American citizen.
This is a drain on our social resources because children of illegal immigrants expand the ranks of lower income Americans that require social services at taxpayer expense.
This puts US-born children of illegal immigrants, American citizens, in a very bad position because they are torn between their home and their parent's home.