I can't for the life of me understand how they don't feel and see the benefits of the balanced position.
Because you have been utilizing this stance for quite some time now and they have not?
Also, consider that you are likely in well-fitting boots that offer good control whereas many of your students feet are probably swimming around in rental boots and they are on dry skis with extremely poor tunes. The heels represent the one sure place of safety and control.
If you were wearing a pair of sloppy over sized shoes and someone taped a pair of 2x4's to them and then pushed you down a small grade, you natural instinct and reaction would be to pull back and fight gravity, not go with it. I guarantee that you would not stand upright with your arms forward and your waist and knees slightly bent in a prepared position. Your initial feeling of loss of control and being taken for a ride would trigger an automatic and instinctual reaction to pull back. Your gut reaction would not be "This is fun", it would be "Holy sh*t !"
When you are new to the sport and have what feels like slippery 2x4's under your feet, you are experiencing new sensations and an intimidating feeling of being out of control on a slippery surface--this is in addition to the trashy rental boots. Everyone has to get over this natural inclination to pull back. It's not until you loosen up enough to not always be pulling back that you will realize the benefit of the athletic balanced stance and have the confidence to put yourself in one. An instructor telling you to do it means nothing. It has to be experienced by the individual before it will finally process. The young and fearless may pick this up relatively quickly in a day or two. With older initiates who start off as adults, it's probably going to take time and possibly years. Instruction will then be beneficial as a tool to undo the bad habits that have become so ingrained and automatic during this defensive phase.
I am not speaking as an instructor but as someone who started skiing seriously as an adult many years back. Once you get comfortable on shallow terrain and appreciate the athletic stance, you tend to lose it when moving up to higher speeds and more challenging terrain. The "Holy Sh*t" factor starts to creep in again and your automatic instinct for survival puts you back into the defensive position that fights gravity. If you want students to appreciate the athletic stance, mellow terrain is therefore the place to do it. Everyone also has their own idea of what constitutes mellow and, as already stated, equipment is a major factor. If the student is in crap boots, it's pointless.
Outside of those who truly have no fear. this pulling back applies to most any skier, IMO. I would wager that you would be hard-pressed to not find terrain or conditions that would not cause a skier to be sufficiently intimidated so as they always remain in this athletic stance. I don't care who it is, if they are sufficiently intimated they WILL lose the athletic stance, if but for brief periods. Everyone has a comfort limit of some kind. It may be trees, 'no-fall' zones, ledges, rocks, etc. Take the most radical and gnar skier and put blindfolds on him/her and ask him to do a back-flip into Corbett's and ski to the base and I GUARANTEE you this skier WILL pull back and will not always be skiing in an athletic balanced stance.