Other than a handful of nut-jobs out there, no, none of us think the undead will rise up and try to eat our brains. "Zombies" is a useful metaphor for whatever societal collapse one might want to be prepared for. For those who consider such things, that could be foreign invasion, viral outbreak, civil war, economic collapse, natural disasters, etc.
If you're prepared for a zombie apocalypse, you're prepared for any of those real things. By focusing on any one of the "real" events, people start arguing over the likelihood of the event instead of focusing on the prepping methods. So if a person says they're preparing for a civil war, someone else will come along and say that won't ever happen and they need to be preparing for a natural disaster instead. Neither poster knows the likelihood of either event, and it's irrelevant. The tools, supplies, and skills needed to survive one are more-or-less equally employed to survive another. It's not a perfect overlap (usually less shooting in a natural disaster vs. civil war for example), but by and large that holds true.
So "zombies" form a common ground that we can all work from. As was posted above, even the CDC has a zombie preparedness guide. If you look closely, it's pretty much the same stuff in their hurricane and earthquake guides. They finally learned that by adding a little fun to their literature, people may actually read them.
You'll also find zombie threads that have little if anything to do with prepping, like The Walking Dead threads. That's just because zombies are fun.