The problem with passwords is that ideally they are long random strings and different for every site. But humans are not good at remembering such passwords, so

The problem with passwords is that ideally they are long random strings and different for every site. But humans are not good at remembering such passwords, so they tend pick shorter passwords and to re-use them on lots of sites. But computers are good at remembering lots of long random strings, so why have we not developed a standard for site log-ons which the browser chooses the passwords and stores it securely for the user?