There are numerous factors to this. First off, the natural motion blur caused by film allows for the brain to track the information better and gives an illusion of fluidity. Games on the other hand render images statically, one by one, often inconsistently. And depending on the motion of the camera in game, the next frame may be dramatically different. (This is partially why some games can run at 24~fps looking smooth, while others look choppy even up to and past 60fps).

And while you are right that folks who played GTA IV, and other games that rendered at a usually smooth 24-30 fps, didn’t often complain about motion sickness, this is a biased sample. The reality is that we know frame rates and frame times are linked to motion sickness. This has been a very prevalent problem with VR headsets that, in which the proximity to the screen exacerbates any issues. But folks playing GTA IV at the time were not likely to be part of the group that was susceptible to the motion sickness induced by low but consistent frame rates.

Compared that fact to now though, where it’s very possible to run games at a higher frame rate, which means that people who would experience motion sickness at lower frame rates can join everyone else in the glorious hobby. Also, if you are having low frame rates on a PC nowadays, it’s more likely to be paired with inconsistent frame rates, increasing the choppy feeling.

Fwiw, just googleing “games cause motion sickness” and adjusting the search date range to '06 to '13 brings up no end of results, including the forms post about GTA IV causing motion sickness for at least one gamer.