CPU cycles are cheap. Every major distribution has a build server. Determining dependencies is easy with proper package manager. Rebuilding all binaries should be as easy as pressing one button, and not take more than a few hours.
If only that were true. There are some packages which take hours to compile by themselves, even on beefy servers.
IIRC, you can go from zero to a complete desktop Gentoo system in 2-4 hours on an i7 desktop. But even this long is admittedly an annoyance most users would not want to endure on a regular basis for a modest increase in security. The main reason people use Gentoo seems to be configurability, not security.
Don't know about GP, but I can think of a few more: KDE, OpenOffice, Firefox (that alone takes about 2 hours on my quad-core Phenom), Chromium.