You may be safe in one area, but if you bactrack there will be zombies, and if you stay too long, a few specials will spawn. Being rushed my be fun to you, but not others, and Valve always tests their games thoroughly.
Having to shoot the zombies in the head and only the head would be a pain, especially in games liek L4D. Yeah, sure it's easy but when the horde comes, it gets more difficult. And when your friend is pinned down and surrounded by the horde, it's even worse.
It's kinda hard to had zombies nibble on your then continue to fight them. I guess the zombies like to kill their pray first. Or maybe they don't want to eat you, just kill you.
Yes you should be rushed a little bit more, but the zombies won't always know where you are. Having to shoot every last one in the head would get old, frustrating, and downright stupid fast. I'm sure companies planned on forcing the player to shoot the zombie in the head, and head only, but during testing they foud that people didn't like it.
It's hard to show zombies eating people, and the chunks being taken out of them. And after that people would be asking how their character is still fighting after they were half eaten!
These elements are left out because it makes the game hard to make and hard to play, and I'm glad they left them out.