Because I am a UI stability freak. I have extreme distaste for people breaking my working environment, so every physical system I own is currently LFS simply so that user-facing components on whose UI I rely can avoid being jumped to versions that change the user interface in major ways. Not udev, as such (that's just very annoying: I have to run it but I don't like to be bothered by it), but things like the desktop environment, for instance. Now maybe I should just be using xfce, but it so happens I'm a KDE 3.x + fvwm user. These days, some people package Trinity, but there was a long time when I was using KDE 3.5.10 and no vaguely up-to-date distro packaged it: there are other major components that I am intentionally far behind the curve on too. At the same time I want or need to use bleeding-edge trunk versions of some things (kernels, toolchains, QEMU, Emacs, Chromium), a lot of which have substantial local patches applied.
There are few distros optimized for stick-in-the-muds who also need to use and patch bleeding edge low-level software, and I can understand that. Nobody else is likely to want to use the same combinations of bleeding-edge stuff as I am, and handling all the possible combinations is a combinatorial explosion. Most people in this position use a distro, locally compile lots of stuff, and try to cope with the much-too-frequent disruptive UI changes. I'm not willing to do that. Sometimes this means I whine about people who assume that everyone uses a distro that someone else maintains (and that downgrades never happen) and that they can break low-level components' compatibility frequently without anyone at all caring. People like me do still exist. We may be a small constituency of control-freakish eccentrics but we are not nonexistent. :)
(As a side note, the reasonably high degree of low-level knowledge that doing this sort of thing requires *did* actually get me a pretty damn good job. So it wasn't a complete waste of time after all!)