The reason I know is because my company made the chips.
They have not been made for 20 years.
Originally they were not just sold as singles by Borland, but also sets of 200 and 300 in cases.

Do you realise how many were made? Over 200,000. Not surprising at all there are tons around. Half of them were still in Borland's inventory when he died and ended up in various dealers hands. I had around 5,000 originally (and still have 1500+ left, although 90% of those are just a couple of values)