>I'm impressed looking that some concept arts are not only arts, but are close to architect's plans. That is not a requisite to do a setting: for example taking again FiM, there is not a clean idea of how is large and big canterlot, or what is the right schematics of Dash house or the old Library of Twilight. Beside remembering that the house have some rooms, like bedrooms or kitchens.
There's a simple explanation why the concept arts of landscapes and buildings have architect feeling to them, complete with plans and scales: Filly Funtasia is 3D. As in, the space of Filly Funtasia has the same number of dimensions as the world we live in (as we perceive it).
This means that the developers can actually construct full environments, with lots of details, and then save them for later use in scenes. This is the same principle as in video games - essentially, the world of Filly Funtasia is a set of levels.
This method works best in 3D and not in 2D because of the camera movement. Moving the camera around is a very welcoming and widely useful trick for making the action on the screen more interesting, so it's used a lot.
However, in 2D animation, in order to shift the viewpoint to another location, the entire scene must be redrawn by the artists, even if said movement is only for a couple of degrees/centimeters. Even if it's drawn on a computer, it still requires some involvement from the artists, and human factor dictates that inconsistencies are there a priori. Of course, the artist may program the scenery to move with the camera, but this blurs the line between 2D and 3D. 3D graphics is an illusion created by calculations.
In 3D, everything is computer-generated, and moving the camera is a simple matter of calculations, with no need to redraw everything from scratch.