A big part of RHEL IS their kernel. They spend a lot of time to optimize/stabilize their releases. Why would they support any other kernel?

It's a tradeoff. There's nothing to stop you from taking a RHEL system and installing the very latest kernel, and there's a good chance that the resulting system would work fine. However, RH the company only supports their own versions, which seems reasonable. Also, and not unrelated, I believe RHEL kernels hold a stable ABI to the point that you can use vendor-provided (out-of-tree) drivers and not have them break on every release, which is something that normal kernels intentionally don't support. So you can use RHEL like any other Linux system, but you lose some of the advantages that made RHEL useful in the first place.

Nothing stops people from using whatever distro they like, instead of RHEL. If they are using RHEL, presuming they’re rational actors, it’s because the RHEL model works for their use case.

You're free to do whatever you want with your Linux distribution, but if you're not using the thing that the vendor supports, they're not going to support it. There's not really any way around it.

Unfortunately the kernel has significant and widespread implications on the system, so it's very hard to figure out how to support a system that is running a kernel version you haven't validated. As a RH customer, the way I handle this kind of thing is usually by replicating the problem on a "stock" system and working with RH support on fixing it on that system --- this is often an important step anyway since, for my own purposes, I need to figure out whether or not I introduced the problem by something I did.