Old medical equipment has value for the same reason why anything old has value. Supply and demand, nostalgia, etc. A lot of these probably where tossed in the trash or scrapped for their aluminum value as most ambulance dealers, ambulance service owners, and fire departments just assumed that no one would want them.
I bought a nice F-W Model 30 from a retiring ambulance dealer within the past year. He had a pile of stretchers that he had already cut up. The one that I bought was next in line to be scrapped. He also had a pile of '80s and '90s van ambulance lights that were destined for the land fill. I didn't want them at any price because you can't even give those away.