I feel like the main problem is that people confuse "discipline" with "abuse", and it's usually the kids who got abused who grow up to be the ones who are most against the idea of any form of physical punishment (assuming they don't internalize it and become abusers themselves).
Any form of punishment can be abusive if taken to excess. Even seemingly benign punishments like getting sent to bed without dinner or being forced to sit in the corner (just look at the psychological effects of solitary confinement in prison - that's pretty much one long "time out").
My parents were willing to spank (or threaten with a belt), but also had the advantage of a) they'd only do it for discipline, not to vent their own anger, b) it was a rare, severe punishment for extreme offenses, and c) they would gauge an appropriate degree of force, to be a deterrent but not so much that it caused significant injury (it's the difference between someone bopping their pet on the nose with a newspaper as opposed to beating the shit out of it).
It also helped that they were rational adults who would explain why punishment was occurring and were relatively consistent with rules and standards - they weren't arbitrarily beating me over minor offenses that weren't offenses yesterday, or constantly getting drunk and hitting me, or having bitter fights with each other and then finishing it off by taking their frustrations out on me. I was fully aware in every single case that it was my own actions that were resulting in punishment, and that it was entirely within my own power to prevent that punishment from occurring.
(As a related aside, they were also willing to consider the reasons behind my behavior and didn't simply punish as the first response to all misbehavior. For instance, in 5th grade I apparently became a rebellious student in school who refused to do work and gave my teachers crap, but it turned out the main problem was that I needed glasses and couldn't see what teachers were writing on the chalk board. Rather than just beat me for being a bad student, my parents figured out what the problem was and fixed it. Meanwhile, the main teacher in question did far more psychological harm to me - without laying a single hand on me - simply by not giving a shit and making no effort to understand why I had become so disaffected. Being forced to sit in a corner as public embarrassment, or to write "I must be quiet in class" (because you were trying to ask the person sitting next to you what it said on the board in a sincere effort to actually be a good student) is the sort of thing that can drive even good kids into no longer giving a shit or actively lashing out in retaliation, if they feel like they're being unfairly punished.)
And that discipline applied effectively and early apparently worked pretty well - I was actually fairly well-behaved as a kid, and so my parents didn't need to constantly spank me. The threat became effective enough, because it wasn't an empty threat.
I firmly believe that the motivation and methodology behind any form of punishment is far more impactful than the punishment itself. I also believe that a bad parent can utterly fucking break a child without touching them even once - the core problem isn't whether or not you're willing to hit but everything else that goes with it.
To be fair, I am far more sympathetic to this explanation than I am towards the people that blanket defend physical punishment of children in brash ways.
Talking to various people, I have found that most people who were physically punished definitely were punished for justifiable reasons, oftentimes. It simply seems to then often bleed over into more instances of unjustifiable reasons. Once again, simply my own experience and talking to people, but yes, we are all using our own circumstantial evidence to form these views.