It is very much a rabbit hole, but a rabbit hole I dive deep into every so often. As you said, so much hard work just lost to the world. WaybackMachine can only do so much.
Last year, I built up a second version of VirtualPetDirectory.com and rebranded it to OnlineGaming.Directory. One of the big issues I had with VPD was we didn't have any "offline storage" of the content, thinking that the owners should manage their own. This was mostly in keeping with "we are a simple directory of information". But now, with OGD, and GameSiteArt.com (linked with OGD) I've started the journey to revive the images as best I can. Of course, I need people's help for that.
I have the assets for Icepets (which is still up and running), Misticpets, Kuepets and Cyopets available to me (I was either the owner, or was given consent by the owner). That being said since the site didn't build a lot of traction, I stopped uploading content to them unfortunately because it was a fairly time consuming process for me.
I've tried to reach out to PaulSonny a few times in regards to Boopets because that was another site I really liked the art for, but similar to the whole development situation with him, where I just got no response unfortunately.
A much more recent loss of a game site is Verpets. My hope is that it is still possible to revive but it may be dead in the water too
I can't explain "why they don't sell off the material", maybe they think it's not worth the hassle. Basically at that point they are probably at the end of the ropes with the site and just want it over. I know that Misticpets is a bit of a special situation. Basically the server is still intact, it's just at a data centre and not turned on (and unable to be turned on remotely), so we are just waiting on a trip there.
I absolutely loved digging a bit and seeing that Mythodreas still was up and running. Unfortunately over the years I lost touch with the owner, I'm hoping that their story is preserved.
The lost content is absolutely brutal. It's my goal to dig as much of it as possible out of the graves that they currently are in to hopefully showcase the work that was done. One of the future plans for GameSiteArt.com was to add in art attributions to the because it's the artists that lose out even more. They used to be able to use the sites as a showcase for their artwork, but when they are gone (which is normally outside of their control) they lose their hard work.