If we're talking about landing off-base, then the sim simply follows real world operational guidelines.
Generally speaking, the widely accepted way to execute an emergency landing is with the landing gear up, especially if landing on terrain that might be rough and uneven, as opposed to a smooth runway.
The reason is simple, the wheels can snag all sorts of stuff and get blocked, bounce, burst tires or even bend and break a strut when landing on a field, which creates unpredictable scenarios (getting bounced up back in the air and the stalling into the ground, breaking a strut on one of the wheels and ending up in a ground loop, etc). Or they might just end up going over a ditch or another uneven feature in the terrain, which will result in the aircraft nosing over with potentially fatal results.
There's lot's of things that can go wrong depending on what might break and no definite way for the pilot to assume which one will happen (if it does). In other words, wheels up emergency landings are more predictable and that's why they are the standard method used: find a flat piece of ground, touch down and the aircraft will skid to a halt.
