Demesnedenoir
Myth Weaver
One typical reason can be: If a prologue is basically a chapter one, it can be called prologue as an indicator that the main character is not involved in this chapter, since there is often the expectation that chapter 1 is the intro to the MC. Can be plenty of reasons, this is one off top of my head.
The concept of a prologue is far different than the concept of a paragraph. For one thing, writers don't put the word "paragraph" in front of their paragraphs. It's the label "prologue" that makes the prologue a bad thing. The actual text of the prologue might be great or not, just as the text of a paragraph might be great or not, with that I agree. And some people might be immune to the psychological effects of the "prologue" label. But in general, according to the comments I've read in numerous venues, prologues labeled as such are less well received than if they had simply been called, "Chapter One." That, to me, makes the use of the word "prologue" at the beginning of the opening material of a book an inherently bad idea.
As for the "real story" starting in the prologue, why was the prologue in that case not called, "Chapter One"? Calling it a prologue if the story really started there makes no sense to me.