> So, whatever the on-disk structure, the motivation here is that from an OS API perspective,

There are multiple layers to the OS API. There is the Resource Manager, which provides the structured view. Underneath it is the File Manager, which gives you a stream of bytes. You can use either API to access the resource fork, and there are reasons why you would use the lower-level API.

One example from the documentation was to provide a backup. For various reasons, it was possible that a resource fork could become corrupt—this is back in the day that macOS had no protected memory (for shame!), disk was slow, and we didn’t use journaling filesystems. Some programs kept around backup copies of whatever file you were working on. If your data was stored in the resource fork, well, there’s an easy way to get a backup… just open the resource fork as a stream of bytes and copy it to another place on disk. You could copy it a data fork, and some people even copied it to a data fork in the same file.

The other main reason you would use the lower-level API is because you are writing a program like MacBinary or Stuffit.

> This in mind, it's curious that we never saw an archive file format that sends the hunks within the resource fork as individual files in the archive beside the data-fork file,

Well, there are advantages and disadvantages to that approach. You can already access resources inside a resource fork inside various archive formats, like MacBinary, AppleDouble, and AppleSingle. But you probably do want to preserve the actual byte stream of the resource fork itself. (And there’s also an undocumented compression format for single resources.)

I am not old enough to know how resource forks were implemented on Mac OS but this is definitely not the case today. Resource forks are implemented (or maybe "emulated" is a better word to use? Not sure how much effort is put into them) as random-access. You can use POSIX APIs to interact with them (using _PATH_RSRCFORKSPEC) and these are typically faster than other interfaces.