If you log in to AWS, then visit another site, your password is still on the clipboard. 1Password clears it out after ~30 seconds, IIRC, but it's still a bit of a risk.
All processes running on your computer can read the clipboard at any time. Many have plugin systems that expose clipboard contents to plugins.
Normally when you use a password manager you have to trust the password manager, the browser and the OS. By copying to clipboard you have to trust every piece of software you ever installed and every update they later got.