Processing power. A movie is not rendered in real time; they use massively powerful "render farms " consisting of hundreds of computers working together to produce the final images and it may still take a long time to produce each frame. For a game, all the images are rendered "on the fly" and it has to be done fast enough to keep the game running smoothly.
As computing power increases, more and more detailed models are being used - just compare some older games to present ones: eg. DooM, 1993: http://i.nextmedia.com.au/news/doomgamei... Quake 2 (1997) and Rage (2011) http://s471.photobucket.com/user/arjuna_... Watch Dogs (2014) http://112.imagebam.com/temporarylink/OO... Things have drastically improved over time - and you also have to allow for increases in screen resolution; When Doom came out, 640 x 480 "VGA" graphics were luxury (I first played it in 320 x 200, "CGA") - now 1920 x 1080 is just about essential, so that also needs around a more than 10x increase in graphics processing power over VGA just to keep the same image quality. "The Last Starfighter" (1984) was the first movie to use full CGi scenes - all the space scenes were computer-generated. It was rendered on a "Cray X-MP", the fastest supercomputer in the world at that time.
If you compare that to present games, I'd say that present games far surpass early movie CGI.
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