(Why 73? It sits in the middle of most road bike geo charts for seat angle, and several brands have noted that there is no reason smaller or larger bikes should depart from it, as there is no data to suggest taller or shorter people need different proportional set back. So it is a good reference line.)
Because they can use the same mold on the rear triangle for all frames and lie to us that it does not matter.
At 6'3 , frame setback is always a really big issue for me on a 73 degree frame. 74 degree STA on a road bike? No chance, the rails would be slammed and I'd need a 150 stem and my weight would still not be balanced over the pedals.
WRT 0.883 or 109% ratios, they are stupid. Crank lengths vary. Foot lengths vary. Pedaling techniques vary as does cleat mounting position. A huge variable is the measurement from the crotch to the floor. Who sits on their tailbone? Tailbones vary a lot. Ischial tuberosity to floor is how it should be measured.
Balance and knee angle are probably better approaches with hand comfort a major consideration for club riders. So, your approach tends to resonate with me but I doubt any formula will work.
I just did a $450 fit. They did not measure a single thing on me. Nothing. Just eyeballs and video analysis.