The creators aren’t necessarily aiming for photorealism. They want their games to look good enough and be entertaining. Adding more realism could make it look worse. Obviously if AI could solve the hard problems like global illumination that’s great. And I’m sure it’ll be possible to e.g easily add more variation to textures by using larger datasets. But changing the tonemapping to a colder palette, or just replacing the asphalt textures with smoother ones?
The creators of GTA didn’t use the sunset tones in the game by some creative mistake or lack of computing power. They obviously went for non-photorealism deliberately! It’s a stylized view of California. It’s like the burger in the commercial doesn’t look like the real one - it looks a bit unrealistically perfect because that’s actually more attractive than the real thing. Realistic and photorealistic are tangentially related.
The comparison here should perhaps be between frames of the game that already attempted to look like the cityscapes dataset.
GTA obviously doesn’t have a ground texture that isn’t smooth because it wouldn’t be possible, or “too warm” palette because it’s too computationally expensive to have the bleak cityscapes tone mapping.
A GTA frame rendered at its current frame cost, with textures the same size (but different) and a customized tone mapping, would look more photorealistic and make the difference to the cityscapes dataset much much smaller.
The authors could likely have done this comparison since they probably have the ability to modify textures and shaders to set tonemapping. That would have separated out the differences that are only artistic from those that are lacking photorealism because of computational expense.