and the other big reason is that apart from the pain of the upgrade XP will run passably on a 1.5Ghz / 500Mb machine especially if you pull some of the start up crap and throttle back your AV settings which W7 will certainly not.
So if you understand and accept the risks and can handle the possible consequences sticking with XP is a perfectly rational decision. As
llondel suggests even after they have lost data some people don't back up. Which suggests that many of them see it as an annoyance or inconvenience not a catastrophe.