noob questions about android updates

[elipsey]

1. why don't (competent) users build android from source as a last resort for updates?
2. do the android sources (or the mainline kernel) include open kernel drivers for most phone hardware, or not?
3. do the android sources include a kernel, the GNU user land, a build configuartion, etc, or are they not really a complete OS or ready to build?
4. if not, where do people actually get android for a particular phone (besides cyanogenmod which doesn't support many phones)?

thanks