Well, they did everything on purpose… with the purpose of satisfying business constraints – shipping a shiny end product within the deadline, and nothing else.

Well, they did everything on purpose… with the purpose of satisfying business constraints – shipping a shiny end product within the deadline, and nothing else. So they quickly repurposed all the iDevice stuff, minimally expanding the boot process to be more open and general-purpose-computer-like. They could've built the SoC differently (with standard Arm components – GIC, SMMU, PL011… – instead of custom ones), could've used UEFI+ACPI, storing the firmware on a typical SPI flash chip, and so on. But that was not done because that stuff doesn't do much for the business objectives of the company, since what's being marketed is the full experience with macOS, not an interoperable standard platform for your favorite Unix. And so they went the easy, lazy, proprietary way.