This is basically a bug with early start year games. They start with a lot of low infrastructure airports, but the growth of infrastructure is about 1/10th of what it should be, in order to catch up with where late game start games are.
GW2 / HaF still has 240 airports in the US as size 1 in 1997. 330 in North America, 300 in Europe
When MT games start, in year 1998, only about 20-30 airports in the US are size 1.
There was a fix, which targets Size 1 airports, but it works too slowly, and it targets airports around the world equally, as opposed to favoring the US and European airports more, because they have demand...