This response answers too much--why did we invent and adopt tar for all other use cases, then? Why do we not still use ar for its original general archiving purpose in any other situation but this one? Obviously there is some reason! I'm curious what that reason is. You can make regular archives with ar today but we don't--"it still works" wasn't good enough reason for any other archive use case. What are the general archiving requirements that don't exist for object archives that meant ar is still fine for this one use case?
Because we care about metadata and robustness for data files, but don't for some automatically generated artifacts that are consumed by the next program anyways? What do you need your static archives to have, that ar doesn't provide?
That is a good point. Although I can only guess, it intuitively makes sense to me why the executable format would more quickly evolve than the archive format which "just works" still.
Why do you find it surprising that the archive format from that time was used to archive a bunch of files?
I wasn't alive but I'm pretty sure ar wasn't only used for this purpose in unix.