Zombies are an impossibility anyway, so any attempt to explain it too much would fall apart in the viewer's mind. They have to be vague about a lot of stuff to keep the willingness to suspend disbelief.
That, and the fact that this takes place in the near future/alternative present, which means that Obamacare could not handle it, and allowed the destruction of the population.
In the 28 Days flick the “zombies” actually did run out of energy after a while and kind of rotted away.