hobodave's answer is most of the way there but I think it understates the importance of protein toxins. With the vast majority of foodborne illnesses, the bacteria aren't particularly harmful at all; what you need to worry about is the protein toxins they produce.
E.Coli - probably the most well-known form of food poisoning along with Salmonella - is actually a harmless bacteria that already lives in your lower intestine. But there is a particular strain of E.Coli, notably O157:H7, that is primarily associated with food poisoning. The reason? It produces what's called a Shiga-like Toxin.
E.Coli contamination is actually dangerous on two fronts. Because the bacteria are so well-adapted to surviving in the human digestive system (as I pointed out earlier, that's their primary habitat), ingesting even a relatively small number of the bacteria will result in them multiplying and producing those toxins in your gut (and the rest of the way down). This is why it normally takes several days for you to feel the effects of this type of food poisoning; that's how long it takes for them to produce the toxins in sufficient quantity for your body to notice.
But they don't need to be in your gut to produce those toxins; a piece of meat at room temperature provides good enough conditions and more than enough raw material for them reproduce and emit those same toxins. So if you leave it sitting out too long, then it really doesn't matter how many bacteria you kill, you are going to end up with E.Coli poisoning fast, because you don't even need to wait for them to produce the toxins; they're already there.
The problem is that you can't "kill" a protein toxin with a brief burst of heat because a protein isn't alive. It's just a protein. The temperatures and times needed to destroy that toxin would be similar to the temperatures and times needed to destroy all of the protein in the food, draining all the nutrition value and quite possibly turning it into a lump of charcoal.
Salmonella seems to be a fountain of misinformation with all sorts of people saying that it doesn't produce toxins. This simply isn't true. Inside the host it produces what's called an AvrA toxin (which isn't "toxic" per se, but allows the bacteria to grow to larger numbers), and some strains can also produce a CdtB toxin, which is highly toxic. (Apparently there's also a similar toxin produced by other strains.) I'll be honest, a lot of the medical mumbo-jumbo is way beyond my ability to comprehend, but it seems that a lot of the public confusion comes from the fact that salmonella can do some nasty things even without the toxins - but that doesn't mean that the toxins themselves can't do plenty of damage even if you manage to kill the bacteria.
The same applies to many other types of dangerous bacteria; C.diptheriae produce the diphtheria toxin, C.botulinum produce the botulinum toxin (botulism); even the infamous mad cow disease was, as far as we know, caused by a protein, not a bacteria, which is why it was able to be transmitted to humans even through cooked beef.
Are protein toxins the only reason why the USDA insists on a maximum 4-hour cumulative danger zone? Probably not. As hobodave says, the more the bacteria multiply, the harder is to kill all of them, even at high temperatures. The figure of 74° C / 165° F that the food agencies give us for poultry is not going to kill exactly 100% of all the bacteria, and if it only kills - I'm just throwing out a number here - 99.999% of them, that may be good enough for relatively fresh poultry but won't be enough if you've got a whole bacterial colony to worry about.
We can only speculate as to exactly what's entailed by the "danger zone" but my guess is that it's actually a combination of statistics, probabilities, and safety margins, which include, but are not limited to, the effects of protein toxins.
Best Answer
Meat being "moist" or "dry" is an oversipmlified description of the texture. It is not a straightforward measure of the amount of liquid, and also, meat is not a sponge.
When people say that a piece of meat is "moist" or "juicy", they mean a specific texture that includes muscle fibers at a very specific stage of denaturation - they have some elasticity when bitten into, providing resistance, but are also relatively easy to crush during mastication. There is also lubrication from cell plasma (which is not pure water, but a mixture of water and proteins, gaining some colloidal properties) and fat. Even if you could produce a squirt or gush of water (or sauce) similar to what would happen if you were to bite on a wet sponge, that wouldn't really contribute to the texture people seek.
Dry meat means meat in which proteins have been denatured to a quite advanced stage. They harden and become chewy, the actin and myosin no longer slide against each other, and I'm not entirely sure what happens to the cell plasma, but I suspect that part of it oozes through the damaged cell walls, and another part seizes due to the dissolved pieces of protein, just like overcooked egg custard seizes. If you have fat left, it will provide some lubrication, but today's meats are produced with extra low amounts of fat, due to customer preference and cost efficiency.
Even if water were to enter the meat in its "dry" state, it won't turn it into a pleasant texture, because, as explained above, this is not about actuall wetness. But also, liquids don't penetrate meat that well. Take, for example, a piece of meat from a stew that has been stored for 2-3 days in the fridge. There will be stew liquid on the surface, but if you wash it, you will find little evidence of that liquid, maybe only within the first 2-3 mm. If you cut it in two and lick it, it won't taste like stew liquid. If you squeeze it, no stew liquid will come squirting out. In short, it turns out that the physical structure of meat with heat-damaged cell walls doesn't really permit for soaking up liquid.
Combine the two effects, and you will find out that most mental models that the public has about meat texture ("moist" having to do with adding moisture, or browning to "seal in the juices") are wrong. What you are trying to do is to get the proteins to the state in which we expect them, and that is achieved with some, but not too much, heat.