There are a few things you have to take into consideration with realistic style maps, and how pb2 differs from real life:
1) Speed. In real life, people creep, crawl, and briefly sprint from place to place taking cover. Physical limitations prevent you from zipping around like Speedy Gonzales on crack. In pb2, you aren't limited by stamina, and to keep things more interesting, you can move considerably faster than in real life -- sometimes in excess of 70mph. Airball bunkers provide almost no cover when you're running around the map that fast.
2) Jump height. In real life, you can't jump 6ft straight up, or double jump off of a box and go 12+feet up in the air. With a realistically scaled map in pball, people spend more time jumping on or over bunkers than running around them. Just for reference: a typical pb2 barrel is about the same size as one of those large cylinder air bunkers. A realistically scaled barrel would be about 32units tall (the size of one of those half-sized boxes).
3) Accuracy. Weapons in pb2 tend to be more accurate than in real life, as do the players using them. You can't afford to play real paintball every day, whereas online it costs nothing to sit down and polish up your skills. If the map is small and open with bunkers providing little cover, the rounds will be very short and not allow much variation. Round starts, and everyone dies before the round start sound even finishes playing. Rinse, repeat.
4) Repitition. Real fields are mostly a flat area with scattered bunkers. Changing the layout on a real field may make a big difference, but in pb2, it's practically all the same because the bunkers provide so little in the way of cover (see above). If you play one map with a realistic layout, you've played them all, pretty much. Combine this with the fact that people put more hours into pb2 than real life, and it can get really dull really quickly.
5) Lag. The more open a map is, the more polygons get drawn, which means lower framerates and choppy gameplay. Also, if lots of people are playing, it can saturate the network bandwidth with all the paintballs, players, and other items flying around the screen.
6) There's already about 300 maps like that. It seems almost everyone who's ever played the game has attempted to make some kind of speedball map, so now we have more maps than players.
Just because they're not in rotation on the servers doesn't mean they don't exist.
I hope that clarifies things.
