Imagine pitching Joust when nobody had heard of it before.

I remember a friend dragging me to the arcade to see what he had found there. It was Q-Bert. Now of course it was a wild game, but he said, "no wait, watch this", proceeded to jump off the blocks, and I heard the mechanical clunk at the bottom of the machine. Someone came up with that just for the F of it...brilliant.

I think for a lot of them, they must have come up with some cool looking sprites and then tried to make a game/story around that.