If you don’t mind me asking, then how do you know the kernel they use is bloated compared to any other kernel? A vast majority of the device-list stuff is loaded only when that device is detected with kernel modules. You aren’t actually running everything from the entire kernel, it just has support for the devices if it does detect them. which is basically the functionality you are asking for, ad-hoc device modules.

Monolithic kernels aren’t “bad”. That’s subjective. Monolithic kernels have measurable and significant performance benefits, over micro kernels. You also gain a massive complexity reduction. Micro kernels, historically, have not been very successful, e.g. Hurd, because that complexity management is extremely difficult. Not impossible, but so far kernel development has favored monolithic kernels not without reason.

If what you say is actually that easy, why wouldn’t all distro’s just do that during the install, and during updates with their package managers? I believe you could do this in Gentoo, but I don’t know if it has measurable benefits beyond what performance tuning for your specific CPU arch would give you. Since none of those devices you aren’t running are consuming any resources beyond the storage space of the kernel.