I am an armchair philosopher and I have no data to support what I'm about to say; however, I remember feeling quite lucky (dare I say even a bit smug)to be in a district that tested kids for GT when entering kindergarten, and then extremely fortunate to have a place in a gifted magnet for my son.
Then I realized that the program was really geared more toward bright high achievers, and that my son was screaming with boredom in Kinder, where I was told that "what we're really teaching here are social skills, so it's OK...don't worry," and then in 1st, when I was told that "he's really not THAT bright you know. I have smarter kids in the class." To FINALLY in 2nd a teacher who new how to differentiate, managed to challenge my child and admitted that he was more than just quite bright.
Now my child is in third grade. This year's teachers are not as adept at differentiation. But they are offering loads of opportunities for independent work beyond the curriculum. AND allowing my child to test out of sections of curriculum and then work on independent projects.
My inexpert conclusion on this? That most kids do not have the maturity to work independently until 3rd grade, so that is when schools test them because they feel comfortable acknowledging the child's giftedness because they can accommodate without spending a fortune and can allow the kids to work independently even if the teacher is not good at differentiation. It's less of a commitment and less of a risk of failure for the school to meet the (acknowledged) needs of the kids.
You might ask why I kept him in a program that was not serving his needs? The answer is that among my large circle of acquaintances I couldn't find a school better than this one. Even if I was willing to pay 25k per year.
So in my area, I would say that gifted education doesn't really start until 3rd grade, even if they are willing to go through the motions before that, because the portion of the curriculum that goes beyond the basic curriculum relies on the children to be able to work independently.
Call me a cynic, but I think that's the real reason.
