Ever wonder why this game heats up GPU's so much?

[Reaujien]

You have no clue how the engine is designed

Quite true - none of us do.

...only what is being displayed

Quite true as well. However, this can lead to deductive reasoning.

If the game is drawing more than it needs to, then the GPU is doing more than it needs to; and the more the GPU is "doing" the hotter it will be.