Because it looks like what you see at the beginning and end game cutscenes and it also shows the pyramid in the background. Does the portal to Mystic Lake lead to a mirage or is there a lake right outside the pyramid it warps to? In that case Wario's intentions would be less about getting out of the pyramid and more about getting all the treasure... but wouldn't Wario notice treasure if it was left outside of the pyramid and why not go to the locations the portals warp to instead?
