In the movie, it is explained that the machines first tried to create an ideal world for the humans to believe in. But it didn't work - the human mind wasn't built for handling an ideal world. For the Matrix to work, it has to be somehow miserable.

When The Matrix was written, the term "gamification" had not yet been invented; the Wachowski brothers probably just didn't think of it. But a game world would presumably have similar problems as a paradise world. People would see through the illusion.

It is of course also needed for storytelling purposes; if the world is enjoyable, then Neo is the person who spoils people's delusions, instead of a man who brings freedom. It would put a different slant on the story.

1) Machines are not our friends.

We are at war with the machines. Not some sissy, slap-fight, don't-pull-my-hair kind of war. We scorched our sky in an attempt to wipe out the machines. Humanity said, "I'd rather never see the sun again than for these machines to continue existing." Now, that may not be a big deal for those living in Seattle, but the sun is pretty important to pretty much everyone else on Earth.

Godwin's law

Bear with me on a quick thought experiment. Imagine that in 1943, Hitler managed to come up with this kind of technology. Human-battery powered incinerators for all types of non-Aryan subjects.

Even with Matrix-like, almost perfect Virtual Reality, can you imagine how many British people would sign up to literally power the holocaust? Americans? French? Pretty much any other country in the world? Sure, you can jump really far, and move really fast, and you're pretty strong. But, at what cost?

Giving yourself up as a battery for good VR is, in essence, becoming a traitor. At that point, you are powering the enemy's war against you. It's a terrible tradoff.

2) The numbers just aren't there

Now, let's assume I'm wrong. Earth is completely filled with morally-bankrupt idiots who can't see the consequences of their actions. There still wouldn't be enough volunteers to create the power that the machines want.

To back this up, I'm going to use the statistics table at the Wikipedia MMORPG comparison page. Assuming that every number in the table is a unique user, and no one plays more than one MMORPG ever (which I'm sure is completely accurate), there are currently approximately 45,080,500 people playing MMORPGs.

Let's say that since the Matrix VR is so awesome, there are 10x as many people willing to join in. This gives us 450,805,000 human-battery volunteers. Which is still only ~6.5% of Earth's population. The machines would take a 95.5% power reduction, while still trying to fight a war, if they only took volunteers as batteries.

None of this takes into account that you are officially giving your life over to this VR. MMORPG players only spend 21 hours a week on gaming. So, it's doubtful that you would even get a 10x increase from current players. It's one thing to spend 12.5% of your life playing a game. It's another to turn over 100% of your life to power the very war that's trying to eradicate you from existence.