Yes, the problem is ARM. How do you want to boot your kernel? There is no BIOS or UEFI or other standardized firmware, that would initialize at least one CPU,

Yes, the problem is ARM. How do you want to boot your kernel? There is no BIOS or UEFI or other standardized firmware, that would initialize at least one CPU, RAM, storage and load your kernel from there. Every ARM implementation boots in different way. After you boot up, how do you enumerate devices that your kernel can use? There is no PCIE or other plug and play bus, that can enumerate attached devices. Your OS has to know what is available. Poke a wrong port and your board freezes. This is the reason, why Device Tree is a thing. Linux kernel does not require all drivers baked in - kernel modules existed for decades already. Monolithic does not mean that everything is one big blob. NT is also monolithic, for example.