In order for public schools to have "pull out gifted programs," or even a "gifted class" with extra enrichment twice a week, they needed to take in a larger number of children than the number of actual extraordinary talents in their schools. An elementary school with 500 students (which is right around the national average) would need 20 students minimum to do a "gifted class," but of course no one wants to put 4th graders with kindergarteners, so now you need four or five grade levels of "gifted class," each with 20 students. Now 100 out of the 500 students are gifted, 20%.
There are a lot more kids in the 80th-90th percentile "gifted" range in these classes than there are the 99.9th+ percentile kids who are genuinely extraordinarily talented and not served well by existing school systems, so "gifted classes" become midwit academies. They don't serve the ultra-bright autist hiding under the desk who might be able to cure cancer someday, they serve the grade grubbers and theater kids.