Legacy. It’s how things used to be done. Just like Unix permissions, shared filesystem, drive letters in the file system root, prefixing urls with the protocol, including security designators in the protocol name…
Be careful to ascribe reason to established common practices; it can lead to tunnel vision. Computing is filled with standards which are nothing more than “whatever the first guy came up with”.
If the alternative was putting the information in some hypothetical file attribute with similar or greater level of support/availability (like for filtering across various search engines and file managers) then I'd agree there's no reason to keep it in the file extension in particular, but I feel the alternative here is just not really having it available in such a way at all (instead just an internal tag particular to the JXL format).
Be careful to ascribe reason to established common practices; it can lead to tunnel vision. Computing is filled with standards which are nothing more than “whatever the first guy came up with”.
https://en.wikipedia.org/wiki/Appeal_to_tradition
Just because metadata is useful doesn’t mean it needs to live in the filename.