The problem is that PG’s ‘recipe’ for Horizon games doesn’t really work for an urban setting, especially not one as tight yet dense as Tokyo. The roads are designed wide to account for any potential scenario they need it for (Showcases, circuit races, point-to-point races, multiplayer convoys, etc.), which mean they’d be far too wide to realistically portray it.
That’s not to say they can’t do it of course; Edinburgh and Guanajuato are very well designed in my opinion, going against the grain of the usual roads found in the rest of the game and allowed for some pretty technical courses. Certainly an improvement over Nice and Surfer’s Paradise. Plus, I think their goal is to find picturesque locales with a lot of scenery on offer. Having a location with the city as the main focus takes away from that a bit.