Why do we wait?

One thing about education reform that strikes me as odd is the fact that so much of it focuses on high school intervention. Why would you not want to intervene earlier - much earlier - to give successful reforms a chance to compound their impact over several years of school, rather than over just a few?

Consider, for the sake of discussion, some reform that raises annual academic achievement by just 5% per year, an effect we were able to replicate each year over the course of a child's remaining school years. If we were to introduce that intervention in the 3rd grade, that child would leave high school at the academic level of a college junior.

Onthe other hand, if some influence reduced annual academic achievement by just 5% per year with a compounded impact, that child would complete high school at the level of a 9th grader.

See this chart to see the impact of such an early start:

Just a 5% change in achievement, assuming it was repeated in each subsequent year, shows a dramatic impact over a long academic life: it's the difference between leaving high school at an academic level between 9.6 - 15.2 years.

In contrast, an identical effect introduced in 9th grade shows only a one-grade level gap between students being accelerated or slowed 5% each year:

Given the dramatic change in course we can effect at the earlier grades - why aren't we focused on reform at that level, rather than when it's too late to make a major difference in the academic paths of students?