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.
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!"