IIRC Marcan mentioned something he found that had been deliberately put into the Mac boot loader that made booting alternative operating systems easier and perhaps making it possible altogether.
That's apparent enough from the fact that you don't need a jailbreak exploit to boot non-Apple-signed kernels on a Mac, unlike iPads with exactly the same silicon. They are intentionally configured differently.
iPads are cheaper than MacBooks and more popular. They'd rather prefer if you bought another one instead of using it indefinitely. The same with smartphones. The answer always has been: I like money!
iPads and MacBooks are architecturally different devices with different purposes (but soon the difference may vanish). People tend to upgrade their phones/tablets more often than their PCs/laptops. Macs aren't locked down because they are designed NOT to be locked down. You can write drivers for macOS (otherwise it couldn't compete with Windows or Linux) but not for iPadOS.
Yes, because if they didn't, the fact that macOS doesn't lock you down in a sandbox means that is more like the entire boot chain would've been jailbroken. The boot chain shared for the most part with iOS devices, where Apple 100% does not want "jailbreaks" because it means App Stores that they don't get to take 15-30% from.
The happy path on the Mac was provided so the talent capable of booting Linux on it could take the happy path that hides all of the stuff Apple would rather not have a bunch of reverse engineers sniffing around.