Android kernels are built differently from PC kernels because unlike a PC where you might upgrade the motherboard, or add/remove hardware that would change what kernel modules are needed (and still expect the PC to boot), a cell phone almost never changes its hardware configuration after installation. If the phone is a Wombat 4s, it'll have X, Y, and Z hardware components and these will never ever change. If it's a Foobar 8000, it'll have X, Y, and Q, and there might be two different sizes of W which aren't even something that requires a driver and those will never change either. Including all the modules that might be present on any device compatible with the same CPU architecture would mean wildly more than 90% of the space used by the kernel/drivers would be completely and utterly wasted (and theres at least four more types of resources that would also be wasted that come to mind). This is not a thing one does with a device expected to run as long as it possibly can with a very limited supply of power and less RAM than pretty much any desktop machine you'd care to use.