I have to agree with the OP, Factorio code is actually pretty complex. Lots of it comes from polish and creating the seamless player experience. It might not be as technically complex as 3D engine, but no AAA developer would care about optimizing the fluid or belt system to behave in a scalable way. It's really well thought out.
(Also, there is a saying that it takes a genius to do something in a simple way. So even if the resulting code might be simple, arriving to that point can be actually pretty complex endeavor.)
Most 3d action games have extremely few gameplay programmers, they just pick an off the shelf engine and then most of the technical work is optimizing the graphics pipeline.
Ive worked on AAA games for the last 8 years, and I made my own 2d games for 5 years before it. My current project has something in the region of 60-70 gameplay programmers, and has been this scale for the last 3 years. Writing gameplay code and engine code are two very different experiences with different constraints. There is plenty of technical work in writing good gameplay code.
(Also, there is a saying that it takes a genius to do something in a simple way. So even if the resulting code might be simple, arriving to that point can be actually pretty complex endeavor.)