Imagine a large pipe going into your TV, and you've got each signal assigned a different level. The topmost level might contain the video, the lower containing sound. For cable, imagine hundreds of 'streams' flowing in, through this narrow pipe. In a perfect world these streams would stay where they were supposed to and the picture would be great. Our world isn't perfect.
Crossover is bad.
Crossover blends the edges of one signal into another. Your TV doesn't quite handle this as well as you'd like, so you get a poorer picture. With hundreds of channels, each with their own information 'streams' pouring into your house, and all around your RF cable from your game machine, you can imagine how much of this crossover is occuring. Which is why we seperate the signals. RGB is godlike because it keeps every signal seperate.