A password booklet works well at home, but it's obviously much less secure if you wanted to sign in to a service while in public on your phone for example. One of the major benefits of a password manager is that your passwords are present, encrypted, on all of the device you need them on. Most people don't only need passwords at home, so the odds of theft or loss of the password book are much higher than your example makes it out to be. If we're talking about an average user, the solution of only sign into services at home isn't really an option.