Because computers don't boot the way they used to in the commodity BIOS era. The boot loader has to cryptographically check that it's valid operating system it's attempting to boot.
Well, Apple could follow industry standards, too. The argument was Apple approves alternate operating systems as evidenced by boot camp. That's demonstrably not true anymore.
That's totally up to Microsoft… they could done a licensing deal with Apple years ago to enable Windows ARM to run natively on Apple Silicon hardware.