Having to manually lookup the site in your manager, copy the password and paste it in the form is too cumbersome.
Right, so most users without an intergrated password manager will just use an easy-to-guess password.
LastPass isn't perfect, but as a system it improves overall web security to a large extent by enabling people to use very-high-entropy passwords.
People who want to copy and paste from Keepass (I do for very high security sites) should keep on doing that. But, for Pete's sake, I hope you're not using the totally insecure X11 clipboard.