The bottleneck is art, from what I understand. It's not trivial to generate enough skinned, rigged, and animated, good-quality 3D models. Even if you can reuse the bulk of your engine code, or even build off an existing game, as in the case of total conversion mods, the art demands have been getting steeper and steeper over the years.
Looking at really prolific game studios, they tend to be in less art-intensive genres. 8/16-bit RPGs, pixel-art platformers, etc.
And the people that really crank out games on a limited staff and art budget tend to reuse the same engines and assets with minor iterations. Think Spiderweb Software[1], or SSI with the Five Star games[2].
[1] http://www.spiderwebsoftware.com/
[2] https://en.wikipedia.org/wiki/Panzer_General