I think it is far-fetched to assume that most groups aren't ever playing any characters that don't have at least one of a 15+ Strength or 18+ Dexterity, which is what is required for the scenario of heavy armor having no penalties and anyone not wearing heavy armor being better off with light armor rather than medium.
I do too, that's why I offered an explanation to describe most characters, not all characters.
You said "To me it seems more that people are operating under the assumption that a score will either be at it's maximum possible value, or it will be 10 or less"
I offered the explanation that basically amounts to "yep, that sounds about right to me, D&D is the kind of game where you'd generally want to max or dump a stat".
My experience is that it isn't at all far-fetched to assume most groups are playing characters that either have 15+ Strength or 18+ Dexterity (assuming they care for armor at all, of course) or plan on getting there as they level up.
Meaning you can well see characters in medium armor, but mostly only as a stop-gap measure, except possibly for the few character classes that are built around that kind of armor (barbarians come to mind).
I guess this boils down to a difference of degree, and anyway, it's beside the issue. The complaint is that medium armor is caught halfway between two maxima, rather than being its own maximum.
Bickering about whether no, few, some, many, most or all characters use medium armor doesn't resolve this issue - only a rules change that adds a third score to optimize against will do that.