Why do so many people equate spanking with lazy Parenting?

It is essential to explain to a child, in an age-appropriate way, what they did wrong, why it was wrong, how they should behave in similar situations, etc. It is equally essential to follow through with punishment for the bad behavior. So if spanking is a Parent’s punishment of choice, how can it be lazy Parenting, provided it is accompanied by the explanation?
Just to clarify, my Parenting style aside, I am neither lazy nor uneducated.

Sounds like very hard work, and very responsible parenting, to me.