The biggest reason is that you need to pull in code you don't have access to (i.e. the DRM media modules, or PatchGuard, which is restricted to only engineers that work on it), as well as the sheer size of all of the localization content to build every language of the OS (remember i18n content isn't just text, it's images that contain text too).
There's no reason to build everything from scratch, it's like working on a patch to e.g. KWrite, and deciding to build the kernel in order to do it. If you're working on a Windows component, you install a daily build so it's close to your equivalent of main/master, write your code, and overwrite the binaries on your test machine / test VM. Your development loop is pretty fast in practice
There's no reason to build everything from scratch, it's like working on a patch to e.g. KWrite, and deciding to build the kernel in order to do it. If you're working on a Windows component, you install a daily build so it's close to your equivalent of main/master, write your code, and overwrite the binaries on your test machine / test VM. Your development loop is pretty fast in practice