The reason to make games (and invest in them) is because there's billions to be made as Nintendo, WoW, and MW2 show.
The problem this gen are two fold - Costs are far too high. There's no PS2. The market is split between three systems and only two are even compatible.
It's okay for big budget games like MW2, GTAIV, Halos, GT5, etc as they'll earn their keep. Occasionally one will flop and that hurts - a lot. It's a big gamble but greed drives companies to make those gambles. Movies do it too.
It's the mid-range games market that really suffering as Wii is a hard system to sell on (most developers don't 'get' the Wii) and HD systems cost too much to develop for. Last gen you could sell 200-500k and make a profit on a mid-range game. Now that same game has to sell twice as much on a smaller user base to be profitable.
Publishers don't really want to make take big gambles all the time but they are having a really hard time finding the balance to make it work. Ubisoft perhaps does it best amongst 3rd parties.