It's not arbitrary it's an extremely popular divide. I didn't land on it, generations of RPG players fairly collectively landed on it. There is a comfort zone for what is and is not magical in a fantasy game. Most of us agree hit points represent a wide array of things, but most of us also agree healing a lot of hitpoints in 6 seconds or less is generally magical in nature, but are comfortable healing a smaller amount in that same time (like second wind) or healing a larger amount over a longer period of time (like hit dice). I also think, for whatever reason, people are more comfortable with granting more temporary hit points in a short period of time even if the net effect in-game is rather similar to healing those same number of hit points.
If your preferences fall outside that majority view, there is nothing at all wrong with that. I think all of us have some preferences which are not the norm and I don't judge anyone for differing on what they prefer in their D&D. But you not being able to fathom how someone can disagree with your preference, that's on you not me. We all need to accept these differences in preferences and work hard to understand how others view things without judging their views as badwrongfun or inferring ill intent or lack of knowledge. All I did was state my preferences. Accept that.