seem to figure out how to make the traffic in the game look halfway decent?

I think it''s just a matter of deciding where to focus all the console's power. If you increase the level of detail on the traffic, you have to make sacrifices elsewhere. This means that they may have to make it so there's less traffic or racers on the screen at once. Or it may have to turn down the detail on the racing cars and environments. Or if they choose not do any of that stuff, the game may have massive frame rate problems. Most developers just see traffic as a less important part of the game since you're racing past it at 200MPH, so they usually don't bother with making it extremely detailed.

for me the game looks good enogh to play for lots of hours