Two sigma https://en.wikipedia.org/wiki/Bloom%27s_2_sigma_problem is not a "marginal" effect. And a chunk of that can be explained by mastery learning, which while still hard to scale up is far easier than individual tutoring as a whole. The educational system is not institutionally incented to effectively pursue these empirically workable approaches. It's a matter of bureaucracy-driven failure, not some immutable fact about the world.
There's even specifically developed approaches to effectively train and educate those who might be often described as 'low I.Q. individuals', such as "direct instruction". Guess what, they get no takeup from the educational establishment, simply because they're (quite wrongly!) perceived as being demeaning for teachers.
The average teacher would rather pretend to educate those who are already highly functionally literate and familiar with educational dynamics (including test taking, of course) and can simply "learn by themselves". Education schools consistently cater to these attitudes and push the same approach.
> You should watch some https://en.wikipedia.org/wiki/Memory_sport competitions, would blow your mind what people are capable of training themselves to do.
What some people are capable of, yes. Does it translate to those people being better at other mental tasks of mental ability? No.