Is it? Why?

If it’s because of the religion of the deceased, there’s an easy fix: take someone who was an atheist.

They aren't taking human remains up there for scientific purposes, it's not any-ashes-will-do, they're taking specific human remains up there because that's what either the dead person asked for or some alive person wants to do with them, and more precisely because they are being paid to.

Nothing's gonna be learned here, it's a vanity project, it's entirely just the idea that "we should do something special" with dead people beyond what is strictly necessary from a public health stand point.

None of that supports your prior assertion that it's "absolutely" a religious effort though. Even if it's pure vanity and 100% wasteful, why should any Earthly religion have an overruling say about it?

"Some post death action to a body beyond what is strictly necessary from a public health standpoint" is not the definition of religious effort though. It has nothing inherent to do with worship, gods, faith, afterlife, or the supernatural anymore than the "belief" if you pay for shipment of something it'll probably make it that even if you're not around to see it and that sounds fun. After all, not everything people do beyond the bare necessities of public health is about religion.

So you skipped my question: "why?", and instead decided to argue against a strawman.

Ironically, that seems to answer my question.