In general there are more geometries to render, for the buildings and the clutter, and more AI to manage. Old gen consoles for example, in RDR 2, are really put to test in Saint Denis, goign constantly under 30 fps. And Saint Denis isn't as complex ad Night City
I know why people think it is, but aside from off-the-cuff speculation, is there that much of a difference? All that water, lush grasses and flowers, all those critters and creatures that have far more AI and presence than the "set dressing" NPCs of Night City. As for St. Denis, it is not as large as Night City, but on my Xbox at least, the graphical fidelity is more detailed and each NPC has a life of their own (not realistic in a large city, which is why the fairer NPC comparison is with the GTA way).