With the kids that arguably could be better served by superselectives (i.e. those who, like @cantkeepawayforeverdescribes, "cannot efficiently be educated in mainstream due to how far their ability lies from the norm") without detriment to the ability profile present in mainstream schools, it doesn't really work that way. For some kids, being in a standard comprehensive school top set is like an average top-set student spending their whole school career in lower or bottom sets. Slow, unrewarding, unengaging, frustrating.
You often see parents on here say that if the kid is that bright, then they can do enrichment in their own time, do self-directed learning, and anyway, a bright kid will do fine anywhere (for "do fine" read "get eight Grade 9s and four A*s", when an education which catered for their abilities might get them a lot further than that, though the qualifications would look the same). But just being academically able doesn't mean a kid is magically self-directed and motivated, and in fact they may have worse study skills and motivation than other children, as they've never had to try that hard.
If you imagine a kid with average abilities going through primary school with everything set at a pace far, far slower than they're capable of, who never has to put in any effort to understand, and then at secondary school has to sit through bottom-set lessons all day every day for years, and can get the maximum grades available to them without applying themselves much, do you think it's going to be easy for them to come home and do lots of extra hours of studying to get them up to the Grade 5s they're actually capable of?
I know that excelling in some subjects doesn't necessarily mean excelling in all, so any "special school for abnormally academically able kids" would have to be able to manage someone who's top 0.5% for essay-writing subjects but falls apart when asked to do algebra, or vice versa, and would definitely have to be able to handle learning difficulties, ASD, and so on. But that's doable if the special school is big enough, and generally the most able kids — those one or maybe two kids out a whole school year where it's obvious to everyone they're the "smart kid" — tend to be able across the board, unless there's specific learning difficulties getting in their way.