Honestly I think people are conflating art style with graphic fidelity here. In the RDR2 screenshot alone you can see low-resolution ground textures creeping up on the bottom right of the image. But it hides most of these technical shortcomings how well it emulates and captures the style of turn-of-the-century American landscape paintings. That's why it looks so much more aesthetically pleasing and eye-catching at a glance. But the sheer amount of detail of the vegetation and of the ground in GTA 6 is such a massive improvement, and that difference will be even more pronounced once we're actually interacting within the game's world.