Below is what support says. (if you like the REST API, comment here, or submit a new support request).
I ran your question past my internal team to see if they may have any other insight we can provide. We looked into it and this does seem to be one of those scenarios where there isn’t exact parity between GraphQL and REST for adding files to the store.
We have got our fileCreate mutation which would put files into the Files area of the store instead of the theme assets directory. I am submitting feedback for this as if we do get enough demand/requests it may be something we can add in the future to REST, however I know there are more resources/support being geared towards GraphQL these days so it may worth looking into these options as well in case you’d be able to make use of the fileCreate/fileUpdate/fileDelete mutations.