Dude, remember, in campaign mode, Shepard wasn't just after Price and Soap. He was trying to wipe out Makarov as well. So fire wasn't concentrated on them. Btw, Ghost and Soap MUST be always present, they are like, THE heros. So you just can't kil em off. However, the other SAS or US army npcs are optional, and thus die like flies.

If you want realism, you'l have to join the US army, Msia Army doesnt see much action.