Why are seven-note scales so popular?

Hi all,

Herman Miller just showed me the strictly proper 7-note scales of 19-
EDO. I'd like to canvas your opinions on why 7 notes seem to be just
about "enough" to fill the range of an octave.

Although I'm well aware of the many 5-note scales used by the world's
different cultures, and have quite a bit of practice in creating and
using scales of varying size, as needed, I'm wondering if there may
be some deep harmonic or timbral acoustical reason, or even a
psychological reason why so many scales are around this size?

On the possibility of psychological limits, I believe the old theory
about our short-term memory being able to handle 7, plus or minus 2,
items of information has recently been discredited - or at least
severely challenged. But I can't quote a source for that information.

Perhaps we are more indoctrinated than we know by the constant
barrage of common-practice music?

Over to you!

Regards,
Yahya