Gamasutra - Three Reasons Why We Buy Those Crazy Steam Bundles. Thanks Mike Martinez.
First, In marketing there's a well worn principle called "the scarcity effect." When something is scarce, it automatically becomes more desirable to us than it would be if it were available everywhere we looked. This "available in limited number" trick shows up everywhere from collectible trading cards to special "limited" editions of new game releases (which usually aren't all that limited in all actuality). Ever noticed a store front that had a "going out of business!" sign in the window for months on end? That's the owners trying to capitalize on the scarcity effect. Buy now, sucker, or it'll be gone!

Koku Gamer - Overcompensating with CG Trailers. Thanks Ant.
However in gaming, CG trailers are normally nonsensical and have little, if anything, to do with the game itself. In fact, video game trailers rarely give any information about the game and are meant to impress visually. Essentially this says that gaming, as an industry or an art, is not good enough alone to impress people so excitement is outsourced by throwing together a CG trailer with graphics of such quality and atmospheres so epic that they would never run on this generation of consoles. This is where the CG trailer’s natural faults will kill it. The reality is that gamers are coming to the point where they feel that if there is no gameplay in the video then they will not spend their money.