Who's going to pay back societies that send off educated, healthy young adults to the US economy? The value invested in individuals does not accrue all at once while they are at university.
It makes zero sense to try and balance the value at level of individuals. It is better to look at it at a larger scale (tens of thousands to millions) and approximate the value of what's coming in and leaving. There is a lot of guess-work involved, especially around future-value. There are no precise control-knobs to get the exact number and caliber of people you want; the best countries can do is set policies and hope for the best, without accounting for second and third order effects.