“there are major American Yeshivas that are not in Brooklyn”
In quality, (which cannot be measured by folks like us) you might have a point.
I think the OP was referring to headcount and /or visibility.
That, and the feed into the “top” EY yeshivos and the crown jewel of the all, BMG, which is for the most part, Brooklynites.
From the view I can see (which is typical NY think), Bklyn is the numbers champ of the yeshiva velt.