The tires are so desirable, even used, that you can make back 40-50% of the cost of new tires selling them on CL or FB marketplace. People just don't like dropping 1K +, but they are ready and willing to drop $500-600. That really takes a chunk out of the new tire costs. So, you stay in newer tires and they get your old ones and everyone is happy. Economics + Intelligence.
I haven't bought new tires for a Jeep since I bought my TJ in 2017.
At the time, I paid $800 for brand new (tread depth measurement indiscernible from brand new) set of 5 32x11.5 KO2's mounted on wheels I liked. Retail on the setup was $1500. They're now on the LJ, not quite 20k miles on them and still going strong. I'll probably be selling them this summer to go to 35's and I bet I can get $5-600 for them. Can't beat driving for 3 years and 20k miles on them for a net $300.
A few weeks ago I needed stock-ish tires for the TJ and paid $425 for a set of 5 Goodyear A/T w/ Kevlar in 31x10.5 ($770 at retail) with <5k miles on them, and again, can't really get a precise enough tread depth measurement to tell they're not brand new.
I don't know what it is with people selling almost-new tires but it's like they lose 40% of their value the second they touch the ground, and then they hold there almost until they're worn out, and I'm happy to work that system.