Depends on the game, but in a game like MW2, your average life span is about 30sec to 1min. Hence what you do in that short amount of time is all the game your really playing. It could also be broken down by engagements, as the more you have, the more likely you are to lose, and start over.
Even in games like TF2 and CSS, you are still playing mini games that last only a minute most of the time. Its just that these mini games change as the level progress, which is why I luv TF2 and CSS.
The point being, if the best part of your game requires you to live beyond the normal lifespan, few people will experience it. Even fewer people will experience it on a regular basis, and the rest will become frustrated before they get to it and quite.