Brhino
...There must be a story here... anyone know it?
Using scales evolved over time, across different subjects and companies that made models or kits.
1/32 goes back to a popular size for toy soldiers, established in the 1890s by a couple of the largest manufacturers in Great Britain. That size is 54mm, and those makers chose to use it, because figures in that size fit well with some popular brands of toy trains. 54mm, as a size, is roughly equivalent to 1/32 as a scale.
But even then, there were variations. One maker measured the size from the soles of a figure's feet to the top of its head, without any hat or headgear. Another chose to measure from the figure's eyes to the soles of its feet. And once we get to plastic models, we can see variations from one maker's model of a subject to the next maker's. We can see variations in a scale within one model company's catalog, too.
The other scales in use have their own somewhat similar origin stories. This one comes from toy cars, or slot cars, or car companies' demo models. That one comes from a common size or scale used by ship model builders, and so on.
So, there's not a rhyme or reason, as there might have been had someone decided, "I'm going to produce models, and the size will be a constantly accurately reduced proportion of the real thing." It evolved via a more or less natural process.
Hope that helps clarify a little!