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It wouldn't be hard to buy something like this: [link] that would allow easy comfortable play when not sitting at a desk
But I guess it does make sense from the money making perspective. Although they could release Sony/Microsoft branded "official" KB/M peripherals to cover that base.
Also, in regards to the "Why not just use a PC?" bit. Consoles are $300-400 whereas a decent gaming rig can run anywhere from $750-1300. So...that's why not :P