Spoken like someone who's never seen Jerry Rice play. Trust me, he was leaps and bounds better than any receiver of his era. In fact, it's arguable the gap between him and the next best receiver of his era is as big as any player at any position has enjoyed in the history of the game.
You misunderstand the nature of the argument. By and large saying someone is the greatest ever at something, at least in sports terms, is saying that when stacked against their peers they were more dominant than any other player against theirs. For instance, in basketball damn near every point guard in the NBA and Div. 1 college could run circles around Bob Cousy in his prime. Yet very few can be considered greater than Coos because of how Coos stacked up to his peers. Get it?