I found this is a cultural thing, e.g. from my personal experience it is not regarded nearly as important in the Netherlands as it is in the United Kingdom.

I agree with you -- in addition, the person to eat the "last piece" has the courage to consume the piece that is both the least fresh and has most possibly been exposed to pathogens from "other piece" eaters.

"First Piece" folks should get more flak as they've destroyed a baked work of art.

My dad frequently eats the last cookie in the box or last piece of cake. He then leaves the container there. It's slightly different than normal "last piece" cult, but really frustrating when you're looking forward to the one piece of cake after you mow the lawn, open the container, and realize there is none left.

I think we have bizarrely different definitions of the word "cult."

It's because the person complaining likely wanted another piece, but now they can't because it's all gone. The "logical" person to blame would be the person who took the last one.

Errmm, simply the fact if someone had the last piece. You can't have one.

Because if you still want a piece after the last piece being taken, you'll have to bother yourself with going to the grocery store to grab a whole?

Maybe this:

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.

Because it's more scarce at that point.

The last pieces will progressively often become more and more important because of a difference between supply and demand.

"And route all supply runs to alternative locations for the next two days. Order cake to be delivered to supply closet in 47 hours."