You have to design a game for every console, however, which isn't as simple as designing and testing games on the PC, then porting them to the 360, which is essentially a PC in its own right as they've done with Gears of War.
If you make a cheap port, the release will flop, so you have to change the code so that it'll work with the system. The PS3 will be vastly different to design for compared to the PC and Xbox, even the Wii, and if they aren't willing to put a significant investment in its design for the console, then the game will flop as a bad port. It's why Oblivion wasn't a launch title, and it's why they have to spend a significant amount of time and money encouraging late adopters to buy a game that they can't do as much with (half the RAM). The PS3 itself isn't living up to anything resembling commercial success, so much to the point that many developers are still confident developing new titles for the PS2, because it already has such a widely established market.
It's not exactly unrealistic to design games to be console-exclusive, or 360/PC-only, because the investment required to develop for widely different platforms may not garner appreciable returns for what could have been invested into an original project.
At this point, Bethesda thinks the potential gains are worth the risk in developing a PS3 title, but it depends entirely one whether the hype machine can sustain itself by the time it reaches the platform.
While consoles may be 50% of the video game market (some estimates put it at more like 3/4ths) you have to look at the consoles as individual platforms and not one all-inclusive gaming platform. Consoles are only unique from PCs concerning the primary modes of play, being gamepads and a couch instead of mice & keyboard and a chair.