They're still around actually, But way less than before. Most XBox/PS3 titles has demos to come with, But most game companies are putting way less time into PC games now than they did before, Unfortunately...
Since consoles are more accessible than PC's, And consoles are trying to become PCs more and more, Game companies would rather spend their time making games more plausible for consoles due to the high amount of console owners, And probably also because piracy is on the loose for the PC.
It's either that, Or game companies love money too much and they know that even if there's no demos around, There's still going to be plenty of people buying the game nonetheless.