I understand what you are saying, but I have counterpoints:
1. Free-floating cameras encourage convoluted terrain. For example, in Oblivion and Skyrim, the primary viewpoint was first person and the camera wouldn't go far from the character. If the terrain were really convoluted, it would be a negative because much of the scenery would be blocked at any time. In BG3, there is no first-person view and the camera can go quite far from the character, so flat terrain would be boring.
2. It is a different philosophy, apparently deliberate, maybe based on the camera angle. The world has all sorts of interesting nooks and crannies that you can get minor treasure from, get good views from, or just have fun finding. Some even reward XP for finding. If you made it an open-world design, you would have to make it much 'larger' internally in order to have the same number of hidden areas and specials. Or you would have to do like Oblivion and add more dungeons, i.e., areas that have a transition to a separate map space. Maze-like designs add replay value (yes, there are other ways to do that).
3. They have deliberately designed alternate routes to use. There are almost always multiple ways to get into and out of each area. Some are obvious, some are not:
You can get from the Druid's Grove to the Risen Road directly (requires Feather Fall). Same for the goblin camp to the Mountain Pass (requires Misty Step). Same for the goblin camp to the fallen temple. And there are at least three ways into the Underdark.