Mostly because players feel more entitled to imagine or narrate what they want - if they want anything at all - in a mindset where a resource is voluntarily spent rather than forcibly taken or lost. You may disagree with that premise too.
I prefer to see the expenditure of hit points as a way to avoid a consequence rather than loss of hit points being the consequence in itself. It's mostly semantics, but it's a semantic that I prefer, even if the end result comes out to be the same. It's far from being a revolutionary concept, but it has worked superbly for us.
I'm not here to convince you of adopting my way of handling hit points, and I don't want to hijack this tread in a lengthy dissertation either. I'm just answering to the subject of the thread...
I'm sure it does work for you, and as combative as this all seems, I'm not trying to dissuade you out of anything.
I'm after all the guy that coined the claim that, "How you think about play, and how you prepare to play, has a bigger impact on the game than the rules of the game.", and I would see this shift in perspective you are describing of seeing hit points as being spent rather than lost as an exceptionally good case in point.
My take on things tend to be fiction first, and what we do in the game is merely bookkeeping to track what happens in the fiction. So, my game tends to have this thought process:
1) "Someone tries to stab someone in the fictional reality, which is a doubtable proposition because the person they want to stab doesn't want to be stabbed."
2) "Fortune smiles on someone, and they succeed (at least partially) in the stabbing attempt."
3) "Someone is now stabbed in the fictional reality."
Both of us note that part #1 depends on whether or not the person being stabbed wants to resist. If a person doesn't want to resist being stabbed, we can just skip directly from the proposition to the fictional result. But to me, your idea takes this idea that a person doesn't want to resist being stabbed and makes it a non-exceptional case. The idea of "spending" is that you've added a theoretical step to the combat where for each proposition you need to call out to the potential side, "Spend or not?" Whereas, the idea that you are losing hit points is simply assuming that not resisting is an exceptional case, and that in general everyone is at all times trying to avoid being stabbed as best as they can.
I don't really see a huge difference in a resource being "voluntarily spent" rather than "forcibly taken" if the consequences of not "voluntarily spending" something is death. I'd argue that mindset doesn't make a big difference, as in the real world if someone said, "Your money or your life", we wouldn't think of that person as handing the wallet over voluntarily. "Your hit points or your life" is pretty darn non-voluntary. I don't think your "semantic shift" has changed anything in this sense. What it has done is caused you to reevaluate how you think about the process of play.
And both of us are empowering players to narrate the stabbing if they desire, both in its attempt, and its result. So no change there either.
What you seem to be doing that is actually different beyond semantics is stake setting, that is to say, "Fortune at the End" rather than "Fortune in the Middle".
In other words, your conceptual shift is being used as justification for this take on process:
1) "Someone tries to stab someone in the fictional reality, which is a doubtable proposition because the person they want to stab doesn't want to be stabbed."
2) "We describe two realities, one where someone is stabbed, and one where they are not."
3) "Fortune smiles on someone (or not) and we choose which reality to choose from above."
What this let a group conceivably do, that you can't do in my game, is describe a combat where you win, but where a participant (whether the GM or the player) is in full control of the outcome. In other words, you let someone predetermine what the reality is prior to them rolling. Someone could propose something that would be treated as an invalid proposition in my game, "I want to stab someone, but I want to be sure to leave them alive.", and in your game since consequences can be specified before the fortune roll, that is a perfectly valid proposition and if they succeed in the fortune step the target will be definitely alive in a way that no one is guaranteed to be if stabbed in my game. In my game, I could only say, "Ok, you want to stab someone but try not to kill them, but understand that fortune may not smile on you, and you may end up getting a result other than your intended one." I might treat this proposition as a sort of stunt, depending on what they are trying to achieve and what they are willing to risk, but there would still be the possibility that they'd roll two natural 20's in a row, and stab the heck out of poor 4 hit point Sir Droll.