I assume it's because stuff like that is easy to whip up quickly and is less likely to get messed up on delivery than some of the more elaborate stuff in hotel restaurants.

It's funny you say that, though. In August 2005 we went on a Holland America cruise to Alaska. I didn't realize that my neck had gotten bigger and I had a nasty surprise when I tried to put on a tie for the dining room on tie night. So we ordered room service. The menu was exactly the same as the dining room, and it came in courses (salad, guy goes away; 10 minutes later he comes back and clears that away, brings us our mains; etc.). Very impressive service and I don't recall seeing something like that in a hotel, where everything comes at once even if the menu is the same.