No-one does real intelligence studies in humans because it's such a third-rail in academia. Think "The Bell Curve". Some scientists seem to believe that intelligence is intrinsic. The analogy with dog breeds, with their different temperaments and capabilities, with individual variation, applies to humans as well.
Some people believe being homosexual is also intrinsic, despite identical twin studies showing contrary evidence. People like simple answers. It’s probably partly genetic, partly environmental. In other words, something worth studying further.
"It is difficult to get a man to understand something, when his salary depends upon his not understanding it!" This generalizes. Good luck funding research into a question to which people really don't want an answer to. There's some valid reasons to not want an answer - eugenics was and is a real thing, and its terrifying. For a while we seemed to skate by on a kind of "gentleman's agreement" to not look into it too much. Just like there was a kind of "gentleman's agreement" to not point out the facts of sexual dimorphism in humans, as a paean to feminism. When feminists start claiming superiority of females to males, or extremist egalitarians start claiming we're all the same, I think an argument could be made that they have invalidated the agreement. Unfortunately.
They've studied it extensively already. Twin adoption studies have conclusively proven that intelligence is mostly genetic with environment playing a small role.
See Richard Haier: IQ Tests, Human Intelligence, and Group Differences | Lex Fridman Podcast https://www.youtube.com/watch?v=hppbxV9C63g