Except that for a site that you visit once a week, let alone once a month or once a year, you haven't memorized it, you've forgotten all about it. It may have taken you an hour of looking around to even FIND the site, you aren't going to remember the password, unless of course it's the password you use everywhere else.
I use XMarks for most of the web.
My problem is when you get into some of these sites -- you have to have a capital, a number and a character. Then they advertise you can access them from a phone app. But you are restricted from saving the password, or even the strange login name that you have to use that is totally separated from your e-mail account or your typical user id.
I have over five credit cards, a mortgage, a car loan, my work's website, more than seven SQL and other forums that I participate in. I also have my own website.
I'm smart enough to group my passwords from financial, to e-mail to forums, etc. But I still have locked myself out so hard that a fin site had to send me a snail-mail to unlock my account.
A little bit of this and a little byte of that can cause bloatware.