Probably because it's both patently untrue and still informative in a way. It would be more precisely stated as "there aren't many atoms left between 5(nm) and 0". If my math is right, 5nm is only a width of about 25 silicon atoms.
1nm is about 5 atoms so I guess the next metric would be the number of atoms. Single atom features would be kind of cool but not buildable with present technology.
It isn't so far off the fin width. And practically, it isn't a terrible scaled density metric. Increasingly bad, but not terribly so.
On the 5nm: it is more useful to think about scaling minima in terms of unit cells. For silicon, 5nm is about 10unit cells, where silicon behaves approximately.bulk-like (like silicon).
When you make it thinner, it is statistically no longer uniform.
The same logic applies everywhere: you need roghly 5nm of fin 5 nm of gate, and 5nm of space: fin pitch is limited at 20nm. For metals it is the same, but worse for resistance reasons, so pitch is limited to about 20-25nm. Gate is even worse.
Even though I think all of this is sort of clearly true, there is so much room for innovation elsewhere. I. Package, stacking, memory....there are 4-5 orders of magnitude out there in power-performance. In silicon!
