1. Actually, Cracked had a very good article for why a zombie "apocalypse" would fizzle within days. The short and skinny is that decomposition is a problem, their food (humans) have guns and even tanks and helicopters, their only means of reproduction puts them against more humans with tanks and helicopters, etc.
Basically think how good any other predator was at extinguishing the human civilization. Leopards for example are fast, are cunning, are incredibly stealthy, etc. Exactly how good were they at collapsing human civilization? Well, not very good, were they?
Zombies on the other hand are slow, stupid, not very agile or stealthy, etc. Where a leopard can sneak up to, say, two dozen metres and then do a lightning-fast sprint to close on you, zombies are invariably depicted as visible from a mile away and lumbering all the way.
2. I would add, there would be very FEW zombies.
Corpses decay beyond any possibility of biological functioning pretty fast. If you want to believe a virus scenario, not a magic scenario (though it would take magic to restart the cells of a rotting corpse anyway,) then you'll need a body in good enough shape to start functioning again. You're not going to be chased down by a pile of disjointed bones.
You also won't want embalmed corpses (which tends to stop any biological activity, which is why it prevents bacteria and fungi from destroying the corpse) or cremated ones.
So let's say a corpse is good enough to raise from the grave for... what? ... a year? Actually even that's a massive overestimation, since after a year all that remains are the bones and teeth, with just traces of tissue on them. If you want any semblance of tissue, we'd be talking those buried in the last couple of weeks. But, ok, let's say a year.
By way of comparison, the average life expectancy (with one significant digit) is about 80 years.
So if everyone were buried, nobody were cremated, nobody were embalmed, etc, and everyone who's in reasonably good condition were to rise at the same time, the undead would be outnumbered by the living some 80 to one. They wouldn't even be a credible threat to the standing army of any modern nation.
But actually, probably most corpses ARE cremated or embalmed, so realistically we'd be talking 7 billion humans against maybe a dozen million zombies. And they wouldn't all rise at the same time, but it would take a while for the contagion to spread. So the total number at any given time might be numbered in thousands or tens of thousands tops.
Far from being some scenario where each guy has to mow down thousands of zombies, actually you'd have to fight a few hundred or even thousands of other people to get to shoot a zombie.
3. Also the "disbelief" isn't much of a saving grace. While people may disbelieve that it's really zombies, we are good with dealing with _humans_ rioting and assaulting the police or army, and have telephones. The first riot squad that finds itself overrun by zombies, would call for help.
Now the guys at the other end might not believe it, but the alternative thing to believe is that some crazy gang of
humans are attacking and killing policemen (and civilians), and were reported even eating the corpses. It's the kind of thing that would get the guns passed around, the SWAT called on scene, and if that fails, next thing comes the army. Though, see above, probably even the SWAT would be enough to finish them off.
There is nothing that needs to be zombie-specific there. We're GOOD at dealing with humans going on murderous rampages. And whether or not we believe that they're really already dead, we're still going to deal with them.
A civilization that simply disbelieves that anything bad can possibly be happening, just wouldn't have made it this far.
4. Finally, since I've been talking about that decay and all, note that even magically raising an army of skeletons wouldn't be much better. It takes decades for the bones to finally become brittle and crumble, but they do just that in the end. By 50 times or so of burial, that skeleton will be in no condition to fight. (And the skeletons of old ladies with osteoporosis, obviously even sooner.)
And that's already being as generous as to assume that they can work without cartilages connecting those bones. Because those decay a lot faster. So I'm already granting that someone or something would raise an army of unconnected bones, somehow.
So again, not only you wouldn't be swamped by everyone who ever lived in the last hundred thousand years, but they'd STILL be outnumbered by the living.
Plus a human skeleton just wouldn't be all that scary in a fight. At about twenty pounds or so, you can blow them away (and likely apart) even with a fire hose
