The reason is. To be evil. Other reasons are just a bonus.
Back on the subject of the staining, I think it's because playing a game is closely related to having a dream. Dreaming is part of the learning process. Playing a game is a similar learning process, relying on much the same brain matter. There's bound to be crosstalk or leakage from one process into the other. Both deposit their results into the same accumulator. Both utilize imagination, experimentation, comparison and, consequentially, emotional modulation.
If there's a state, similar to the hypnogogic state, in which we are highly suggestive, occurring during game play, then let me coin a name for that state here. It is the hypnogaming state. It's neuro-linguistic implications are profound.
Peace.