A few reasons come to mind...
It is the most efficient: your body has momentum down the slope and all you do is remove what's between the body and the down the slope direction, at the end of the turn: the outside leg. The other option is to do a hard hit, send the COM flying over the long leg. I think why most think it's easier the other way, it's because it's more comfortable mentally, since it's like running, walking etc.
Affords the most control and the most options. Extend to release will disconnect you from the snow and you cannot re-engage the skis until the hips fall down enough. Imagine you ski 3d snow and you mistimed an apex or just saw some weird terrain coming up on your line. If your default and the only option is extending, you have very few options to change the line as needed. With flexion, you can end the turn when needed, transition as needed (with full edge control) and re-engage as needed. You can pressure the edges at any time between flat or 0 to 90 degrees edge angle, you don't have to wait for the hips to come back within pressure range for the edging angle
since they're there already!!
If you see anyone ski bumps or 3d snow with fluidity, that's all it is: full command of flexion and absorption and edge control, to alter line at will. Those small adjustments make the difference between a jackass jamming every bottom and a fluid skier, those guys we all admire.
What I do now is, when I ski 3D snow and something weird shows up, I simply drop even deeper as soon as possible, because that gives me the most options. I can extend fully and hop over the thing or not. I can flip the edges and dig hard bending the ski at will, or not. I can do everything. If I was stuck unable to "drop" and my only move was extending up, I would be screwed, because your only option is well, up. And when you come "down" well, you come down and apply pressure. There is no way around it, because, you know: gravity! With flexion, gravity is your friend.
Floating is the technical concept. You can't get floating with extension, because you're either connected to the snow on a long let (so bounced around) or disconnected when the hips are high.
And a lot more biomechanical issues (you can't tip the feet with a long leg etc)... but I typed a lot today...
But I normally find this question funny, because this is the kind of images you will see from great skiers:
etc... you'll never see an extender" in 3D snow looking good... they may think they are, but they're not.
So forget all that technical groomer mambo jambo and try to comprehend this: if you can't flex to release, you won't ski well all mountain, ever. Period. And you'll never be a good... let alone a great skier, ever. Period.
Once you get it and can do it without effort... why go back to extending, unless you really need to?
The problem is that it's not easy to get. There is a certain order of things and very few can coach it - as we can see: many are not even aware they are doing it naturally, they've learned it as a side-effect, thus they cannot control it and/or do it "at will", because they're not aware of it... they either "have it" or they don't... I learned it logically and specifically, so I am totally aware logically about my tactical options and can choose the response.
This is also my big gripe with the way it's "taught" by not teaching it. The reason a lot of otherwise great FIS racers struggle skiing bumps and all-mountain as a great skier is because they never learned to flex as a thing, so they can't do it at will. They learned it subconciously. as a reaction to a gate or a line or a rut, some external cue which will trigger it, like
@Skitechniek proved in the other thread. But since they don't "own it" they will struggle to use it in other environments they're not accustomed to, like bumps etc (although a lot of them grew up in bumps, but those are their own external cues). It's almost like the coaching establishment created coaching strategies, to promptly forget all about it and just coach course after course!
By nature of being an engineer, I'm cursed to remember all the coaching courses I took and all that stuff they taught us about internal and external cues, questioning, guided discovery, decision training... but oddly, there's very few out there that actually use it effectively, to create great skiers. They generally just create good machines that react to gates being in the way...
It's a longer "hard skills" vs "soft skills" discussion if you want to go there... but this is also a reason why racer's skiing is often poo-poed on "for lack of variety" and in a way, for good reason...