This question has been asked many, many times on Smogon's Forums, as well on Pokemon Showdown. The reason for this strange occurrence is because, well, you're playing Random Battles. It's random. That's just what it is. You might get Nasty Plot and three physical moves. It's just random. If you want viable Pokemon / items / moves, play Battle Factory which is the same as Random Battle, except it is more viable. As for Air Balloon Rotom, it's just an old joke.
However, the team that make the sets for Random Battles have been told about this, and a lot of people have complained, so we can just hope they fix it. But for now, you might be better off playing Battle Factory if you want viable sets.
Hope I helped.