People were jumping down my throat yesterday because I was saying that it wasn't surprising that Dead Rising 3 had a shit framerate. Launch games, by and large, are unoptimized and rushed. We remember the great ones... Resistance, Soul Caliber, etc... but the vast, vast majority of launch games on consoles look and play like absolute garbage within 18 months of the system's launch. That's been true in every 3D gen and it will be true in this one.
And this won't be the last game you hear about being 720p. Maybe at some point 1080p/60fps will be the norm but you're in for a helluva shock if you think that's at launch. Almost everything is upscaled.
If you're asking why... it's simple: the tools. Yes, these machines are like PCs, but on the PC most development is about brute force. You have to deal with a wide array of hardware so you don't program specifically to anything - you just use the added power that people have as brute force. That's why a CPU/GPU combo that you can buy today that are much higher spec than consoles will not be able to run anything in 3-4 years... because no one is designing for them specifically.
You can't brute force a console that has the specs that these consoles have. They will be able to do amazing, amazing things over time as people learn the system. But getting tools later and using your PC build means you are going to get mid range PC type output until developers get more time with system and develop to its strengths. Which first parties always have the jump on and is why Forza and Killzone will be head and shoulders above the rest of the launch games.