Per se, it's actually not. There are thousands and thousands of hobby-level kernels floating around. Many university courses actually include making your own simple kernel.

The big issue is that the kernel is the core of the whole ecosystem. Everything builds upon it. So if you build a new kernel, you pretty much need to rebuild everything built on top of it.

As a bad comparison, imagine you came up with a genious new shape for a car fuel hose nozzle. You know, the thing you plug into your car to refuel it. Designing a new nozzle is easy. Getting it made isn't much harder either. Retrofitting billions of cars to work with that new shape is an almost impossible amount of work. So while making a new nozzle is no problem at all, actually implementing it is almost impossible.

The same holds true for the kernel. Making "a kernel" isn't a big issue. Getting it to work with all PCs with all their diverse hardware and software is close to impossible.

The Linux kernel and the drivers running in it easily have billions of work hours invested into it, and still it doesn't work perfectly with every piece of hardware you might have in your PC.