It's because they know you will buy it anyway. Established IPs.
I don't know if you could count the betas as a demo... Well, you get those for halo.

I just can't wait until Gears of Wars comes out. You know, the RTS spinoff of gears of war.
Now that's going to mislead alot of people, lol.

Demos are designed to sell a game, and some games, a demo just wouldn't do it justice.

Demos make me not want to buy games

Some games sheer awsomesauceness can not be experienced in a demo. The strength of Halo 3 is in it's multi-player experience.

In similar ways, some demos conveniently cover up, the shittyness that turns out to be the actual game (heres looking at you Too Human) (IMO).

Well it would lose sales.

Considering the sales (just about to pass 10 million, and just passed 5 million) I think its a simple matter of its not needed (and a waste of resources). Not to mention theres always a few people that don't like a demo and don't buy the game.

That's actually an interesting question. I've never really thought about it.

I guess it's because people have such a certain idea of what a Halo game will be already that a demo isn't necessary to convince people. Ditto Gears 2 (can't explain gears 1 though).

I think we may see a Halo demo when the main series (FPS) evolves again (like Uncharted with the addition of multiplayer player). I guess it's a genuine system seller, and I think if MS charged for a multiplayer demo (not just adding it as a bonus with another game) it'd sell well as well (like Prologue).

The bigger your game is, the less you need a demo to sell it. Demos make great sense for small niche games, unknown firsts and games that just doesn't sell very well.

But what purpose would a Gear of War demo really serve? Everybody knows about the game, they know how it looks, what it's about. The risk with releasing a demo for games as heavily frontloaded as Gears of War or Halo 3 is that the people would have bought the game day 1 just because of the hype will try the game and not like it.

You could look at it like this, 10% of those who try a demo (I made this figure up) is going to buy the game. The demo is good if less than 10% of those who tried it would have bought the game anyway. If you have a big game, the amount ready to buy the game before trying the demo would be much higher, and it's much more difficult to make it a good business move.

They don't really need it

Sometimes demo's sell games, sometimes they stop you buying them.

I wasn't massively pumped for either Need for Speed: Carbon or Bioshock, but the demo's saw me purchasing both. With regard to the latter, it turned out to be one of the best games I have ever played.