Framerate is contigent on CPU cycles, not ram. To use a book analogy, graphics cards and ram draws the pages, cpu flips the pages. In this case, it flips it 30 pages per second.
Pixel cost for Killzone at 1920x1080x30 is 62208000
Pixel cost for Forza at 1920x1080x60 is 124416000
It's why with Call of Duty, they sacrificed resolution for frame rate.
Call of Duty MW2 was 600p or 1024x600x60 = 36864000
Any game that actually ran at 720p at 30fps like most games this gen were 1280x720x30 for 27648000 for pixel cost
There isn't a midway point in frame rate, it's 30 or 60. If all of these others games are running 1080p/60fps on the system, why couldn't it? Because that 1080p/30fps, they can add better graphics because they don't have to flip the pages as often than at 60fps.