Why exactly is that? The fact that you end up in the back seat seems to be the consensus, but no-one has been able to explain clearly why that is... Anyone?
It's pretty simple, really. The vitally important flexing-extending movement patterns of skiing are specific to the sport, and they represent a skill that must be learned. Without skis or ski boots--in "normal" shoes--when you bend down low to pick up something off the floor, your heels will lift and your ankles will bend to a degree limited only by their range of motion. Both of these movements (heels lifting and ankles "dorsiflexing") move everything above them forward, compensating for the hips moving back when the knees bend, and allowing you to keep your balance.
With stiff, snug ski boots that limit ankle movement and ski bindings that lock your heels to your skis, the same knee bending movements would move everything back unless you compensate with other forward movements. Those movements can come from the hips, the spine, or the arms (or any combination) bending or reaching forward. They are not particularly difficult, but they are "new" movements specific to skiing. The sooner we learn and adopt these ski-specific flexing and extending movements, the quicker our skiing progress will be.
With stiff, snug boots, beginners typically simply don't flex much--prompting the ubiquitous and time-honored advice to "bend ze knees." They simply cannot bend their knees and keep their balance without learning new movements higher up in the "kinetic chain." And if they hit a bump or something that causes the knees to bend and absorb the impact, the stiff boots throw them quickly into the proverbial "back seat," for the reasons I described above.
"Soft" boots are somewhat more forgiving for beginners because (combined with a typically looser fit), they allow more "normal" ankle movement. They may allow beginners to keep their balance fore-and-aft more easily before they learn their new ski-specific movement patterns. But of course, getting away with errors can hinder learning as well, and even contribute to learning bad habits.
Stiffer boots are critical to skiing for several reasons. Evolved along with our short little feet, ankles are not strong enough to deal with the increased leverage of long skis. We need to regulate the pressure along the full length of the skis--sometimes forward, sometimes back, usually right in the center--maintaining balance throughout a full range of "vertical" motion, even as dramatically varying terrain and speeds constantly conspire to threaten that fore-aft balance. So we need both the added strength and support ski boots give to our ankles, and new, sophisticated combinations of knee, hip, spine, and arm movements. In my opinion, for anyone who wants to learn to ski well, the sooner you get yourself into a high-performance, well-fitted and well-adjusted boot, the quicker you'll learn the new movements without reinforcing bad habits.
That does not mean getting the stiffest race boot you can find! Some ankle flex is necessary, both to forgive the inevitable error and to absorb the small micro-shocks of terrain irregularities, "washboards," and the like, as well as to provide the ultimate precision "fine-tuning" to fore-aft balance. It's notable that downhill racers (highest speed, longest turns, emphasis on gliding speed) typically use somewhat softer boots than slalom and GS racers, because the softer boots allow their skis to glide better and faster.
There is a lot of mythology about ski boots and the role of ankle flex and boot flex in performance skiing. Many skiers still insist that we must be able to flex our ankles deeply as part of creating ski performance. Without getting into too much side-tracking detail, I'll just ask a question I've often asked: How do skis respond to "boot flex"? The answer, of course, is that they don't. They respond to pressure, including fore-aft pressure shifts, and as skiers, the more precisely and accurately we can regulate that pressure, the better. With the right learned skills, we can flex and extend through our full range, manage the dramatic forces of high-performance turns, and absorb large moguls, while maintaining fore-aft balance throughout. This requires the ability to find fore-aft "neutral" in our boot cuffs at all times, regardless of the degree of knee flexion or extension. If I need to "flex my boots" every time I bend my knees, I will be extremely limited in my ability to balance, flex, extend, and regulate fore-aft pressure.
Get good boots and learn the right movements!
Best regards,
Bob Barnes