There is this whole "It is not a proper meal unless there is a bowl of rice" kind of thinking. So you get rice with the potatoes that come with your Nikujaga, the mashed potatoes, steak and fried potatoes, etc. But then there is that completely arbitrary line that you do not get a bowl of rice with your noodles (unless it is for dunking into the leftover soup of your Ramen or Tsukemen), pasta, burger, etc.

I love Japanese food culture to bits, but figuring out when a bowl of rice is vital for a meal to even be considered a meal and when it is not is still a mystery to me after observing my wife and in-laws for more than a decade. Having been taught the "food pyramid" [1] line of thinking as a child, it just makes no sense to this gaijin how we can not substitute one source of carbohydrates with another, but I suppose that is what culture is for you.

[1]: https://en.wikipedia.org/wiki/Food_Guide_Pyramid