Spanking does not teach a child why their actions were wrong, so why does any rational person expect spanking to be an effective tool in discipline?
Sending a child to their room doesn't teach them why their actions were wrong either. Nor does taking away their toys or any other form of discipline.
Each of those things teaches the child that their action is
unacceptable. But none of them teaches
why it is unacceptable. The why, in each and every case, must be provided separately. Spanking is no different in that respect from time-outs. Time-outs don't magically provide a reason, and a child is unlikley to just discern a reason on their own, unprompted.
You have a very stilted and biased approach to this. You keep approaching the entire process as if spanking is a thing in isolation, where a child is beat mercilessly, experiencing significant amounts of pain, and is given no reasoning or explanation... but when non-physical means of punishment are discussed, then you assume small steps, with very light or ramping approaches, with full explanations and discussion.
Why do you assume that non-physical approaches to discipline occur as part of an entire suite encompassing many aspects of teaching, but spanking occurs in isolation with no other efforts on the part of the parent?
Everyone in this thread who has not been anti-spanking has stressed the importance of a well-reasoned, thoughtful, and caring approach that incorporates explanations and love as well as spankings.