Errrrm.... They most definitely won't just go for an off the shelf part. No console GPU/CPU is exactly the same archetecturally to any off the shelf desktop or laptop part, despite the fallacies that people like to float around the internet. For the console platform holders to even be able to control the rights to their console components they will employ the likes of ATI and NVidea etc to design for them a CUSTOM chip. The developement of such is not a quick-tings process that only takes a few weeks to whip together a design from an existing PC chip. You'd be very naive to think so.
Not only does the hardware need time for development, the software layers also need to be written. We can thank MS that from this gen onwards, gone are the days when platform holders like Sony can simply throw out a new box without any dev tools, sparse documentation, poorly slapped together devkits. These things also need to be developed and that takes a considerable amount of time and effort.
I don't think you really realise how much actually goes into the development of a new console...