I mean, you've just invented eugenics. Restrict the number of children being born that would not help the economy out. The government did do this once upon a time, but there are tradeoffs like riots you have to take into account. The difference between immigrants and babies is the immigrants can't riot within the US, so the US has no need to cater to their desires. However, immigrants that create more jobs than they take, or raise wages and innovation do have a pretty easy time immigrating to the US (e.g. investor and o1/e1 visas).