Keep in mind that the shutter speed on the camera filming will generally be equal to the amount of time the frame would be displayed on the screen for a movie. This causes very accurate motion blur in film, where as in most cases for games each frame is very distinct from the next making them much easier distinguish. Fake motion blur techniques can sometimes mask this, but generally looks really fake...
Also, constant frame rates are usually perceived as much smoother looking than a constantly changing one. For a movie seeing 24FPS all the time seems smooth because you get used to the way that movies look. If you are running from a closed in indoor scene on a PC game that's running at 60FPS and you run out through a door where an open vista battle is going on where you drop to 20FPS you really notice the drop.
Add in to this that frames on a PC generally are used in sync with the monitor refresh rate so ever single frame is on the screen for different periods of time in cases where your frame rate being supplied doesn't match some multiple of the screens refresh rate. As an example, 20FPS on a 60Hz screen will just 'feel' smoother than 24FPS on a 60Hz screen if your Vsync is on because in 20FPS each frame is displayed for 48ms (3 screen refreshes) where as in the 24FPS scenario where some frames will be on the screen different amounts of time. You don't notice this as much, but it makes it just seem off in some way. This is the main reason for 120Hz TV's, which can display 24p 30p and 60p data all in even number of screen cycles where a 60hz TV has a judder effect when displaying 24p content. Again, you don't really notice it as much, but it just feels/looks wrong when you are seeing it.
Edit: as a visual reference, try this: http://boallen.com/fps-compare.html