dumb question from a newbie about hand rankings

I did a search and I haven't really seen this addressed-perhaps someone would assist/link/answer

it has been my understanding that the hand rankings in poker were/are determined by their relative scarcity- the rare the hand-the higher it's ranking-is this correct?

if so, maybe someone could explain to me how and why four of a kind is not the highest ranked hand, rather than the commonly accepted and agreed upon straight flush-with the royal being the highest-

there are 10 straight flushes per suit-using the ace for both high and low-yet there are only thirteen quads-
even allowing for the royals(4) that still leaves 36 straight flushes as compared to the 13 quads-making
the four of a kind much rarer by a factor of almost 3x.

how did the hand rankings come to be agreed upon, and more importantly, why so?