Everytime I hear the argument "just use Unity or Unreal Engine it is much easier" I have to laugh. Such a big part of the rpg maker community is not an actual programmer but a event scripter. Without wanting to insult those people (because you can still create great games this way) but that is simply the same as asking a regular preschooler to explain the Heisenberg uncertainty principle. It is just not feasible.
Of course that is also no reason to work on a 3D maker, there are very valid reasons why this is not done, but this "just use unity" arguments are not helpful at all for most people.
Actually, Unreal Engine includes Blueprint, which lets you 'program' using events/bubbles rather than writing code. It's not as fast as writing in C++, but as long as you don't need to make overhauls to the engine to implement functionality, it works well and has most of the same core functionality as the C++ game logic.
As far as a 3D RPG Maker? Aside from the difficulty in content generation, the price would be extremely prohibitive. Previous RPG Makers released close to or within the 90-150USD range, and that's with 2D assets. 3D assets involve often work on the model, the rigging, the texture, and the skinning- a library of assets close to the versatility of the 2D RPG Maker RTP would probably set you anywhere from 500USD to a over 1000USD, depending on the complexity of the assets, including or excluding the cost of the software itself. What's the problem here? RPG Maker, at least in Glorious Japan, is a niche product. The boxed copies are something you find hidden in a corner of Yodobashi, far from prying eyes. Competent hobbyists that use computers in Japan is a niche- even in University, a lot of people were left clueless when computers were involved, and even my university, a fairly well known one in Kanto (hint, one of the founders was James Hepburn, who set up the form of romanization you see used frequently outside of Japan), still took handwritten papers (the only students who usually typed up papers were the linguistics/English language department's) for submission. That niche of competent hobbyist computer users that enjoy making JRPGs probably aren't going to be willing to pay that much for an engine.