I think there's also an element of "ownership" to the kernel for many of the OG maintainers. It's their "baby". And as every parent knows, you would do a lot more for your babies than for anyone else.
It may well be that same sense of ownership creates an "inner circle" effect that is a barrier to entry for creating more such developers.
The inner circle effect is absolutely real. Some projects get rid of it with very loose commit-bit policies, but even getting to that point can be painful. There are software projects I'd like to contribute to and have (I think) reasonable ideas about, but no way in hell am I running the gauntlet of the communities involved.
It may well be that same sense of ownership creates an "inner circle" effect that is a barrier to entry for creating more such developers.