Based on my own personal experience as a skier matching that description - it's also because while learning to ski, you're generally so horribly back seat most of the time that getting forward starts to become the #1 priority. I was astonished when I took my first lesson this season and was told i am frequently TOO forward at the ends of my turns - I'd heard so much emphasis on getting forward that I actually had no idea it was possible to be too far forward. I actually didn't really believe my instructor until he took a video of my skiing to show me. I couldn't say what role my own boots play in this. I suppose stiffer boots make it easier to just lean into the tongue to balance. What do you mean by on-the-slope boot fit with trail maps?
I assess the student's range of motion with a few drills. Your stance, both amount of flexation and how far apart your feet are factors of range of motion. We find the optimum stance and provides the most efficient range of motion.
At that point I want the boot cuff to have about equal pressure - so that your lower leg rests about in the middle - equal pressure throughout. If not, we shim the boot with trail maps, dial in the correct stance, and you see the boot fitter after we're done skiing.
In all my lessons, even never evers, we focus on correct stance. The stance varies in very few applications. Like any sport, like Martial Arts, if your stance is not consistent, not powerful, not the index and ultimate source of truth that you can always rely on, you are not likely not skiing efficiently.
Skiing is all about your relationship with gravity. It is about keeping your center of mass over your base of support. Stance is where it all starts.
Have a good night.