Why is it still not possible to cut files to paste them elsewhere in Catalina?
Because the Pasteboard, to which all data is Cut, is inherently transient and unsafe. If you could Cut a document in the Finder, that document would only reside in the Pasteboard, and any new Copy/Cut action would cause it to be lost.
You could never Cut a file in Pre-OS X Mac operating systems.
However, you can achieve the same result by Copying (CMD+C), and then Moving (Option+CMD+V).