Excel does not actually do a simple clipboard copy. It copies the cell references and when you paste, it adjusts the cell references and formulae and puts them in the destination. If something changes the structure of the spreadsheet between copy and paste, it cannot guarantee that the pasted formulae will still work. For that reason, it clears the copy buffer.

It’s a complex problem to manage all of the cell references in a dynamically changing spreadsheet and early on the Excel team chose to keep it simple.

You can see a version of this if you do a cut > paste. The source does not move to the clipboard, it just gets highlighted. When you paste, Excel does a move operation that adjusts the references on the fly.