How often do you really expect a customer is grabbing multiple pairs of the same size/model/color of shoe to try on? Maybe it's just where I shop but it's rare that there's even more than one example of the size I need in the style/color I want to allow such a scenario to occur.
All the time? I don’t know if it is still true, but at one point Zappos had free returns. I know a few women who would buy a range of sizes, keep the one that fits, return the rest.
There was a Seinfeld joke to this effect that men just take whatever clothing, even if it barely fits. Women are likely to be choosier and insist on a good match. Some might intentionally mix and match shoe sizes if one foot is bigger than the other (leaving the store with a mismatched pair)
I looked (briefly) online for evidence of the factory claim and found none.
GP didn't give enough context ("MY shoes are weird so I extrapolated" vs "I know this is true; here's proof."), but the casual one-off scenario is absolutely a showroom level of "random happens".
Babysitting all those individual shoes and styles and sizes seems ... expensive, logistically.
In fact, if this were a thing, I'd imagine they'd start offloading them as though it were a cool feature. "Want to mix-n-match shoes? buy just a left!"
