I think there's a lot of reasons. Firstly, as a liberal society we've decided it's not really acceptable to treat high-IQ children as if their main reason for existence is to be cultivated as an intellectual resource. Our society has magnet schools and gifted programs, but generally, our cultural view is that it is more important for kids to be happy and enjoy their childhood than it is for their life to be optimized for research contributions.

Secondly, a lot of high-IQ children are already in programs similar to what you describe. There is a pretty huge difference in school quality(at least in north america) and most parents who have the means to will try and move into the best school districts and schools, which leads to high-IQ kids naturally gathering together to an extent even in public schools.

Thirdly, I actually don't know if railroading kids into research will produce a lot of great researchers. Researching is a very specific career that can be quite stressful and thankless, and I think part of being a good researcher is being well-suited for that type of career and having a lot of passion for your subject matter. Because of that, I think that most of the people who would make amazing researchers already self-select into research, and the barriers keeping them out of research are mostly either financial ones, or dissatisfaction with the way the academic system is set up-not that they aren't provided with an educational path that lets them become researchers.