But then the question would shift to: Why wouldn’t a AAAA cell have the same energy density of a AA cell? Remember that the argument is about energy density, not total energy.
By the way, I think that fsh’s reply is a convincing answer: it’s the extra packaging that kills the density.
The current collector also takes up relatively more volume, on top of the packaging (the wall thickness would be the same, and it's not a trivial part of the volume of small cell batteries).
But then the question would shift to: Why wouldn’t a AAAA cell have the same energy density of a AA cell? Remember that the argument is about energy density, not total energy.
By the way, I think that fsh’s reply is a convincing answer: it’s the extra packaging that kills the density.