Why don't all big games have demo's?

So I just got done playing the Mass Effect 2 demo and I was blown away. I had no idea the shooting in that game was that good. I am a Halo and WoW player and I really don't dabble in much else aside from the occasional sport game or RPG. 
 
If Mass Effect 2 would have released the demo alongside the games release I would have bought the game day 1. I just got back from a local game store and got it used for 29.99. That's 60 bucks the developer won't be getting from me that I would have gladly given them had they allowed me to test the game out.
 
On an slightly related side note I was able to pick up ME1 for 9.99. I thought that was a fair price for a used 360 game.   

Guess the lesson learned here is that if you are going to produce a game, you might as well make a demo for it if Xbox will allow you. Would have definitely made me get this over FF13.
 
 

Demo's are usually pretty expensive to make plus they have to really nail the feel of the final product. I guess this puts a lot of developers and publishers off the idea.

" Demo's are usually pretty expensive to make plus they have to really nail the feel of the final product. I guess this puts a lot of developers and publishers off the idea. "

Some demos just dont demo well, how would a fallout new vegas demo work?

A lot of games, like RPG's, don't demo well. Some demos make the game seem like something it's not. Making demos also take time away from the developers, when they could have spent that time polishing the game, so in a way, demos are a waste of time.
 
That's what I've always thought of demos at least.

Open world games (Fallout, GTA) And RPG games don't demo well. 
It's never a good idea to limit an open world game with a demo (or a timer for that matter, that's even dumber) 

I'd be willing to bet that Mass Effect 2 demo was worth every penny. Me, my sisters BF, and his sister all got the game after we played the demo. 3 people that I know personally. Might cost a little to make the demo, but it is cost effective. Gotta spend money to make money. All the commercials in the world couldn't make me buy that game, but one 30 minute demo sold me almost instantly.

I totally feel you, especially being a PC guy it's pretty rough. Getting in with On-Live has been the only way I got to try the new Splinter Cell and Assassin's Creed 2. 
 
Gotta pray for those quick-looks!

There was actually a mass effect 2 demo recently released on steam :P

When the press plays an early demo, they always go in with the thought that the game has time to get better.  You cannot risk that with the consumer if you have a demo that is substandard.

" @iShayman: @Exed: @FluxWaveZ:  But nearly all games have demos for E3 and gaming conventions, couldn't they  use those? Actually games do improve alot normally between E3 for example so that might not give the best impression "

" I would say bigger games that have a good chance of selling i would think they feel a demo really won't help them as much or maybe even hurt them. When a big game comes out the hype train will  give the game alot of sells. If there was a demo first it could be a good chance it may turn people away because it not as good as the hype and they would move along"