Apparently gameplay videos of GTA 6 leaked

Why does it barely look better than the game that came out 9 years ago

Because it's the mechanics that take the time to do, and only when they're done do you apply the visual polish. If you did it the other way around then you'd be wasting a lot of time and money. The bits I watched were NPC debugging and bone animation reactions, plus some state machine stuff for the hold-up NPCs.

The visuals of a game fade quite quickly, in "wow" factor. It's the core game loop, the story, and the physics engine that are centre stage and all of that is done with low-poly stand-ins.