I noticed the same thing - when I bought mine 2.5 years ago there weren't a lot going around, but still 3 for sale within 2 hours drive of me.
I think they are a few reasons there aren't as many for sale. Firstly there just aren't as many. They've rusted out or been crashed beyond repair. I'd say with parts prices being so high a few that might have been repairable probably got parted instead of repaired.
Also, I think a lot of the guys (and girls) who own them now are a bit older, maybe have more disposable income/more stability in their lives and therefore are less likely to need/want to sell them. If you look at how many Ford Escorts there are for sale you'll see a similar pattern. The people that have them are in no hurry to part with them. And the lure of getting big money isn't there like it is with older muscle cars, etc. where prices are going up a crazy amount
It also seems that as the owners mature a lot less of them are ending up bashed against stuff as drift cars, and more seem to be lobbing up to other, more traditional driving events, or as you've said, being held onto by "true collectors" who want to preserve them