Archive-by-VM has been attempted from time to time. In the case of VFX there isn't a single server, but many different workstations, servers, render blades, etc. So preserving the entire pipeline in amber is a challenge.

And, somebody has to pay for it. The producers of the current project won't allocate any of their budget to preserve something for a (hypothetical) future sequel/re-release - that is somebody else's problem. The only places that I know of that do a decent job with archiving are animation houses like Pixar that own their own IP.

its a question of cost. Sure you can keep a pipeline frozen, but thats very expensive, and there is little need to do it.

its just cheaper and simpler to re-do it from the clean footage, or perhaps get some of the original models.

Possibly but I believe some software packages use a render server to farm the heavy lifting out to