A similar question I have is why use a browser extension for your password manager? I use 1Password as well, and if I need a password I just head into 1Password Mini. It's convenient enough, simple, and completely eliminates this attack vector.
Because it's extremely convenient. When this bug was first reported a few days ago, I disabled the Lastpass browser extension and started copying and pasting passwords from the vault, basically as you describe. It's been a very noticeable inconvenience, to the extent that I probably would have turned it back on soon even without a fix.
I'd argue convenience isn't really the point of password managers. Extremely convenient would be using the same password everywhere or letting your browser save them.
I've never used LastPass, but 1Password Mini is pretty convenient. It's always ready to go in the menu bar, and once you enter your password it stays unlocked for a good while.
