2015 was at night + rain, both make the game graphics "look" better and while in nfs 2015 it's night they can use higher graphics/models for the things you can actually see, and while it's maybe 1/4th of what you would see at day like in payback you can optimize those 25% far more,
a while ago i saw those nfs 2015 mods for day, it didn't look good at all.
but i have to agree from some perspective payback don't look that much better than rivals, but so does forza etc, the main difference imo is that these games have more green landscape which also makes the game look bette/more alive, while an empty desert makes every game look worse/less alive.