Wasn't the reason given in 28 Days that the zombies have a shelf life? But Mike brings up a relevant difference between the lumbering and sprinting varieties: the former places the focus on the survivor's mental state -- will he finally just give in to the trudge of current existence? -- whereas the latter is more about just trying to stay alive long enough for the next action scene. Dawn of the Dead wouldn't really work with the sprinters.
a satire that understands satire is different that black comedy
I always thought Heathers was both.
Man, I gave up 20 minutes in, just couldn't take it. Everything here was already better covered in Running Scared (not the Billy Crystal one), but it wasn't necessarily meant to be a satire, which makes it all the more perfect.