Because they cannot just put whatever they want in the game. Even though they got contracts to use the team name and logo (and for players if their is some type of union, I know Madden has to deal with the NFLPA) it might not have included the right to recreate the stadium, since
they have focused most of stadium recreations in areas they know for a fact it might get more purchases because "St. James' Park is in the game and I like Newcastle". And it is a very difficult task to create a stadium, not to mention adding in how fans, weather, scenery has to be added, and allowing us to download it wouldn't work because they cant create it as stated above, they dont have the licensing for it.
The best video game example I have it Football Manager, some of the leagues are licensed so things like team logos, team kits, etc are in the game (MLS I think is one) whereas others (such as the EPL) is not fully licensed and only a few teams have team logos, teams kits, etc. Just because SI got Man U to agree to let them use their stuff and not Chelsea doesnt mean they wont do their best with what they got.
However I would at least like one or two stadiums for every league instead of default looking ones, make it memorable or maybe even put any stadiums used for National Team games. On FIFA 10 for PS2 I know that the Home Depot Center was at least in the game so it is not like they have not tried.