I've always been baffled at the 'last piece' cult. That piece is no more important than any other piece of the cake. What's with all the emotional baggage surrounding it?
Last beer though...
"First Piece" folks should get more flak as they've destroyed a baked work of art.
https://en.wikipedia.org/wiki/Simulation_heuristic
AFAIU, when something bad happens to us, we focus our regret on the smallest/nearest action that could have been undone to change the outcome. So in this case, the bad thing being that you wanted a piece and there are none left--it's natural to blame whomever ate the last piece, because it's easier to imagine an alternate reality where the last piece wasn't eaten, than an alternate reality with some earlier piece not being eaten and everybody subsequently encountering a cake with one additional piece.