Indeed, but an internal trade route is getting twice as much, because you are on both ends of the route, and it doesn't benefit an opposing leader, and it's safer because it will only travel through your own territory (yes you can get the fence for that, but it takes a while). Same for Stations. For example, there's that station that gives extra production. Why would you do a trade route with it, when doing a trade route with your own cities gives just as much production to two cities, and you get extra food to boot! Etc. If you compare with BNW, internal trade routes only gave either food or production, and only to one of the two cities. They're roughly four times as strong here, and they aren't even limited nation-wide. No idea why they decided such a strong buff was needed.