On another note... don't plenty of juices contain some amount of alcohol? Might the alcohol escaping into the air have anything to do with this? I dunno...thinking out loud and it's rather late.
I think that has a lot to do with it. Burning alcohol fumes are unpleasant to inhale. Ever had a fresh juice take your breath away first couple vapes? Ever smelled alcohol fumes from a Bunsen burner? It's not something you would want to vape or even inhale except off of a snifter of brandy perhaps. Another reason why they burn the alcohol off of food that has alcohol added to it during cooking. They want the flavor of the wine/liqueur, without the fumes.
Another thought is what e-liquid may go thru during transport. How many times has it been hot and cold in 48 or more hours. This alone could cause instabilities that need to settle out at a constant temperature for a spell. I can't imagine subjecting beer or wine to such extremes and still having it taste right fresh off the truck.
Do e-liquids contain preservatives or other stabilizers? I know some flavor concentrates have a gum acacia base instead of alcohol found in typical flavor extracts and are made to be heat stable, but I don't know what effect alcohol would have on it after the fact. Typically, at least with organics, top flavor producers typically avoid alcohol as a solvent with their extracts designated at least for higher heat applications such as baking perhaps. I think vaping, at least at the point of contact with the coil, qualifies as a high heat application. I am willing to bet that the evaporation ( by letting it "steep") of any added alcohol, helps to return these flavors to be able to withstand the higher temperatures associated with vaporization.