Working for one system allows them to specifically target the hardware much better.
Then besides that, there's the games that have taken 5 years to come out for PS3--after that much time it better look awesome. And then as far as games like Uncharted--this will be the third game they've made for the system, which means that they know by now how to work with the system well.
On the other side, just compare Gears of War to Gears of War 3--there's big graphical changes.
That's one argument for extending the console lifecycle. If a company released a game when the console first launches, they only have enough time to get one more game out for the system before it gets replaced by new hardware. For other developers it's even worse, they only get one game out for the system before it all changes again. That's why at this point in time there are so many games with graphics better than what the consoles first had, since now the developers have had a good amount of time to work with the systems.