Putting fans in charge of official game development is a heavily risky investment. Making a 2D game like Mania is one thing, but handing off production of a large scale 3D title to fans is heavily unlikely, if such a thing were to go wrong it could heavily damage the reputation of the overall franchise, and as such cut into it's profitability long term. Making such a thing is a heavily complicated endeavor.
Most fan games are composed largely of re-used assets. It's not very often a 3D fan game appears made entirely of original assets that can at all compare to the quality of assets made by a proper team of professionals. I love the idea of fan games acting as inspiration for official devs to draw from to improve their products, but going so far as to hand over the responsibility of making a full scale large budget game to fans would be a heavily risky and reckless endeavor.
It's much more sensible, at least in my eyes, to hire talented fan game devs (Emphasis on talented, not someone who plops a model into Unreal Engine and turns up bloom) to work alongside the other professionals.