There is still a market for vinyl, albeit limted. And even today that produces rarities.
For my Soul Intention label I have no more than 500 copies of each release pressed up. And of the latest release a number of them were badly warped, so unsaleable. This makes the release in general rarer than intended. Also, limited run promo copies (I get a maximum of 20 done per release) are by default "rare".
I used to be involved in another label many years back, and the unsold stock was taking up space a few years ago - so I binned them
I'm sure this behaviour by other label owners has contributed greatly to the rarity of some records. If you have something sitting around that didn't sell, it can easily perceived as worthless and therefore easy to get rid of.
Phil