If all i wanted is a challenge then I would be studying comlicated math on the einstein level until i get some sort of epiphany or maybe I'd even enjoy learning programming.
I don't enjoy programming even though it's challenging, I only like the reward of making something you're interested in with programming... as in a cool game, not just any game.
In every single way the game is the challenge and the reward is the ending. Whatever happens in between, like getting a new weapon for completing a level is obsolete after you are done with the game. Off course the game needs to be fun to engage in the challenge but that was not the question since for some people racing games are fun, for others online FPS and there are even people that are finding fun at studying math up to the einstein level.