Who knows it seems mad to be, but for example people who had a 4090 and wanted a 5090 are happy to spend whatever to get the best, so if they sold their 4090 and are using a temporary card, they don't want to wait months using said card, or could have been doing a new build and had been told to wait till the 50 series released, and again didn't want to wait months for availability to improve whilst the rest of their system is unusable without the gpu.
I can sort of get the demand for the 5090 though, as that card is for people who have a **** ton of spare money and don't care how much they spend of it on their hobby.
The demand for a used 4090 and 5080/5070ti is just completely odd though.