Files and folders in Sphere can only be "moved" within the same workspace. And with good reason, because moving them to another workspace might increase the visibility of the files to people NOT invited in the source workspace.
Nonetheless, it would be a valuable feature to do that as sometimes the amount of workspaces becomes large. But a authorization protocol (allowing only the "owner" of the workspace to do it for example) seems reasonable and possible.
Another solution to the issue of "too many workspaces" (in software terms a "cluttered namespace") would be an archiving feature where it is POSSIBLE to recall something no longer actively used, but that it is not (by default) displayed.