For the same reason that fast food burgers are often dry :)
When beef hits high heat, the juices go in the opposite direction. This is why when pan-frying a steak, you know it's close to medium rare once you see the juices start to come through the top. The fat in burgers does the same thing, it moves away from the heat source, and back into the rest of the meat. That's just what happens when meat cooks and constricts.
Many fast food joints use a double grill (you pull an inverted flat top down on top of the burger on the flat top). Whoops, there goes practically everything moist in the 3 or so minutes they cook them.
Using steam, you make this a much gentler process. As the patty is cooking from the top and bottom and sides at once, but with a more gentle heat, the fat (and some juices) come out instead of going to hide on the other side of the patty that isn't on a heat source.