TF2 movie in the works . . .

It did too for me, but I thought it's duration would be closer to 1 hour.

Indeed.

If you've only got 15 minutes of story, then making it any longer than 15 minutes is a very bad idea.

On the other hand, Valve time

Every Story is interesting
And Red And Blu's back story is Good.

Because, in just a few minutes of total runtime, the first 8 Meet the Team videos told a better story and had more awesome action scenes than almost every movie ever created.

The same reason you care about the backstory of every game-figure. It adds to the immersion of the game.
Probably also the same reason you care about the backstory of comic book characters, other film's characters, and, well, characters in general.
This also extends to other people; you want to know about the guy you've just befriended. Sure, the past is the past and all, but the past is what shapes you (or other characters).

If what you're insinuating is that "they're just random FPS characters", then I agree, however. It's not like I'm interested in the background story of terrorist #243 (from Counter Strike) or similar.
But that's where Valve went out and made it interesting: by actually adding a backstory to each of the characters, you get to know why they're so different.