Why do football teams like running up the middle?

Football teams do run outside. Certainly if they run up the middle every single time it will be difficult! But the problem with running outside is twofold:

• The back must run much further (horizontally), giving more time for the defensive backs and other defenders to meet the ball carrier
• Running vertically allows the linemen to block more effectively. While some linemen are skilled at pulling and blocking wide, it requires more athleticism

One other note that came to me later while researching this some more: running up the middle sets up play-action passes. You can't really set up a play action pass on a run wide - you can set up the run option that way, but not the play action pass. Play action passes are the most efficient possible kind of play for most teams, and so they run up the middle to set them up. See this Football Outsiders post for more details.

It's also worth noting that running in general is a losing proposition - as the FO post from the last paragraph notes, many teams would be better off passing a lot more than they do now.