I think that one reason is that cameras back then were sold almost as inheritable items. People kept them forever and expected them to last forever. There was no constant upgrade cycle, so the purchase was seen as a photographic investment.
It's why many would engrave their cameras with identifying information, they never thought they's sell them. I see so many used/old cameras with the original owner's name/DL #, SS# on them!
W/re to SLRs, stuff like Nikkormats could be now bought for under $40. Yet they sold for about $800 back in the day. I bought a Nikon F3 in 1989 for $1000. That would be $2100 today. And yet I bought a perfect/like new one for $250 recently.
In the year 2000 I paid $1845 for my Leica M6 TTL. Using the linked inflation calculator, that would equate to $2,789.87 today. Generally speaking, I tend to take very good care of my possessions (not just cameras). In addition, my M6 sat largely unused for many years until I began using it once again 3 years ago. Given the condition that it is in, and that I kept al of the original packaging (I'm not a collector but did so out of habit as I do the same thing with my hifi gear for example) it would now sell for more than the adjusted value.
I am used to my possessions losing value over time, but when it comes to my M6 or my acoustic guitars or my car, all are currently appreciating in value. In the case of the camera and the car, the appreciation has been enough that it makes me somewhat uncomfortable to use these items and that drives me crazy. As such, I've contemplated selling at times in the past, yet I still get a lot of enjoyment of using them and so they stay. There is definitely something to be said for old stuff that is of high quality that just works and is of little value. Case in point, I've begun writing letters once again and bought an old typewriter for this purpose. It still looks like new and cost very little money, but is actually an amazing machine that brings me a lot of joy. I imagine that it will continue to do so for as long as I own and use it, something that can't be said for so many things today in the age of planned obsolescence.