[ocdtrekkie]

Well, for one, for most of the life of your browser, it took no security for anyone staring at your desktop screen to read plaintext versions of all your passwords. (Yes, physical access is complete access and all, but there used to not even be a casual attempt to prevent someone from stopping by your PC when you stepped away having a look at your passwords quickly.)

Later versions of Chrome, IIRC, will trigger a UAC prompt on Windows before displaying passwords, or something similar.

It's also generally been trivial for software to mine saved passwords from all browsers. I'm not fond of password managers personally, I prefer outright memorization, but password managers generally at least try to keep their contents secure, usually.