Normally its just pre-alpha versions of the game that are shown off at E3 etc, there are few demos for games these days as developers preffer to invest the time into the game.
a demo doesnt usually require significant resources, its normally just something thats already been developed and used for testing in house, and then released as a demo. Games are typically developed in pieces and then those pieces are put together for the full game, so a demo would just typically be one of those pieces on its own.
The alternative is to supply the full game but with a time restriction.
I think its more to do with arrogance and wanting people to buy blind, most released videos tend to show very limited gameplay, they usually cutscene focused and when they do show gameplay its in short bursts jumping around so hard to get a grip of what the game is like. My guess at some point a business decision has been made that showing gameplay or allowing gameplay prior to purchase loses sales so its no longer common practice. They dont care if someone hates the game after buying it as at that point they have your cash. This when tied in with retailer exclusive pre order bonuses probably has a huge impact on sales. As people are pressured to buying the game before launch meaning they cant see others play it on youtube etc, get friend's opinions etc. to get that pre order bonus. Notice also how those reviewing the game officially ign etc. get the game before launch but are not allowed to release their reviews early.
BT Infinity 2 Since Dec 2012 -
BQM