In 1970, Bryce DeWitt wrote a paper in physics popularizing and he labeled it the many worlds interpretation of quantum mechanics. For this particular set of questions about dividing up the wave function of a big system, it has never been pressing because people thought they knew what the subdivisions were. This inverse problem of starting with a combined system and asking how to divide it up isn't one that has been very pressing so people don't pay attention to it. I think as we push forward into understanding the foundations of quantum mechanics and quantum gravity, people will pay more attention to them.