$.02...
When it comes to guitars, "rare" might be better taken in context. Regardless of how many were originally built, IMHO rarity is qualified by the number of them that are currently available for sale at any given point. The number of original condition '65 Fender basses (e.g.) for sale is limited, moreso with every passing year.
If 100K Fender "instruments" were made in 1965 (I don't doubt that stat), and with the generally agreed upon ratio of 1 bass for every 10 guitars sold, that leaves ~ 10K basses built that year. Deduct the mandolins, violins, and whatever other oddball Fender creations from the 100K total "Fender instruments", and we're apparently left with less than 10K basses for that year? Is that a lot? I bet it was in 1965 but because that number is finite, and while the number of potential buyers has grown exponentially over the decades, an orig '65 Fender bass qualifies as rare for a lotta people.
When compared to the "other" major guitar makers, whose combined efforts to eke out a little slice of Fender's bass market share that year, amounts to a hill of beans, '65 Fenders could seem abundant. In reality, they're not. I suspect it's just that orig condition '65 Rics and T-birds (e.g.) are way harder to source. Cheers.