Why do you go through teammates in L4D?

Just why? I played the PC demo and I always wondered? WHY? Can't they be physical, existing people where they take up space? WHY?Why do you go through teammates in L4D?
Because then one player could stand in a narrow hallway and block his teammates from advancing, griefing without incurring any of the negative reputation that comes from other events such as friendly fire. That wouldn't be any fun, now, would it? I like the current system, where players take up a little space, but you can still advance past them.  Why do you go through teammates in L4D?
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Why do they say ''you're filling out a form'' when you're really filling in a form? Why does 7/11 have locks if it's open 24/7? Why do they call them ''apartments'' if they're stuck together? Why are they called ''buildings'' if they are already built? Why do you park in a driveway, and drive in a parkway? Why do they tell you to ''dust the house'' when what you're trying to do is get rid of the dust? Etcetera...But yeah, I like it better this way.
really the people in the game with u aren't physical thats weird
Its an arcade shooter... I don't see how going through teammates has an effect on what little ''realism'' it has in the first place.
[QUOTE=''Albanian_Killa'']Just why? I played the PC demo and I always wondered? WHY? Can't they be physical, existing people where they take up space? WHY?[/QUOTE] it would break the game
considering how fast paced the action is, it would be really annoying to always be running into teammates.