Queens off the board equals Endgame

I have heard and read in several places, from several GMs that when the Queens are off the board it is the Endgame.  What is the reason for this?  Is there a certain combination of pieces that must be off the board in order for the Queens' removal to be considered the begining of the Endgame?  Or is simply trading queens in the opening automatically move the game into the Endgame?  Can someone answer this please?

If one pawn is off the second rank, it's endgame.

There is no clear delineation for the transition to an endgame. I disagree, however, that the absence of queens on the board necessarily makes the position an endgame, as there many queenless positions that are quite complex. Moreover, the absence of queens is not a necessary condition for the position to be an endgame (e.g., K+Q vsK+Q endgames). To paraphrase Justice Stewart, I know an endgame when I see it.

I think the endgame starts with King activity