> And you add the bootloader dead last, even though it’s the first thing you see when you boot a Linux machine?

That actually makes perfect sense. If you did the bootloader first, how are you going to build it? How do you know what to boot from which filesystem on what device? By the time you've got your root filesystem complete, it's easy to tell.

If you do things step by step incrementally, you keep having to go further back in the process. Starting over repeatedly gets tedious, boring and frustrating. And it'll take ages to finish the complete OS.