Since seat tube angle has little effect on max power output, why not adjust the reach with saddle setback? According to this article, a seat tube angle between 65 and 110° has little effect on max power output. At most, it varied 1%.
https://www.me.utexas.edu/~neptune/Papers/jab26%284%29.pdf

Adjusting the setback would affect the effective STA. The reason for a more forward seat position seems to be an aerodynamic position.

If it works, it can allow wrong sized bikes to fit. If the bike is too large, the reach would be too long so to decrease it, the seat is moved forward. If the bike is two sizes too large, one might flip the seat post to move the seat even further forward. The opposite is true for a bike that's too small.

Is it possible it's rarely done because it affects aesthetics, preferred postures, safety reasons, or weight distribution?
Bike fitting isn't just about maximising peak power output. In fact it's rarely even the priority. The study that you've linked only touches on absolute max power without looking at any other factors which may actually be more important for most cyclists. This study is only really relevant to track sprinters who have to do a max power effort, almost as soon as the race starts.

The most important thing for most riders is comfort. For recreational riders, this is because they enjoy riding more when comfortable. For racers, this is also important, because it becomes very difficult to race for hours on end if your bike fit is uncomfortable. 

The study doesn't look into the effect of saddle position over the course of a longer effort. You might find that while the difference is only 1% in a max effort, it could be far more important when looking at the power that can be produced for hours on end.

Bear in mind that elite track sprinters will spend very little time on their bikes at all. They always set it up for their very short race efforts, because it doesn't matter to them how they feel on their bike after 20 km of riding, because they don't need to do that. Be wary of applying the findings of this study to cycling in general, because track sprint is a very specific discipline with very different requirements which don't apply to the rest of cycling.