There are a couple of factors that I think are driving used airplane prices. First one, and the 500 pound gorilla is the low production numbers for the last 40 years. If you look at the
GAMA Annual report, you'll see that since 1982, the average number of piston airplanes delivered by U. S. makers is maybe 1500 units per year. Compare that to the 1960s and 1970s where the average was closer to 10,000. Second one, which can be found in the
FAA Airmen Statistics, is that the number of sport, private, and commercial pilots has stopped declining. Third one, and I don't have data to support this, is that I suspect the size of the genuinely airworthy production fleet is shrinking, with the exception of Cirrus. Some airplanes get crashed, some get destroyed in storms, some get exported, and more than a few get left out to rot. Last one, and the one we see here, is that there are a few aircraft types that are market favorites: Cessna 172 and 182, Bonanza, and Tiger come to mind.
I'm not sure why the market has heated up lately, maybe the coronavirus has triggered a YOLO moment in lot of those who were thinking about getting their private ticket. As far as flying activity goes, the best proxy I can think of for the number of hours flown is avgas consumption, which you can find
here. It's down fairly significantly since Covid hit us in March, down by almost a quarter from last year.