I think it's because when you copy something to clipboard you are also putting a note in your short term memory about it. Our short term memory isn't very good,

I think it's because when you copy something to clipboard you are also putting a note in your short term memory about it. Our short term memory isn't very good, with only a few slots for storage. A way around this could be for a mini-image of the contents of the clipboard to be displayed in a corner of the screen somewhere.