I propose that the main reason for this is because browsers (and password-storing applications in general) have practically zero support for OpenID. It's all handled by hand. It's all in your head, and nowhere else.
Sorta reminds me of, oh... browsers before password managers. Far far more people used only a couple username/password combinations, or stored their ones in a text file somewhere. Now that managers are integrated, and secure external ones exist, it's viable to actually be safer with your logins, and many more people do so. And now I rely so heavily on my password manager, I only know a couple key points-of-entry where its data is stored - the rest of my passwords are all randomly generated.
OpenID is essentially the next step, and it's precisely the same end-user problem. Imagine if browsers supported it natively via a profile-like system; pick a profile, and every site you've associated with it is immediately logged in for you. You'd be able to handle multiple people using your computer easily - just enter a password to use that profile, and everything can be switched for you. No need to launch that password manager for every site, you sign in once and everything is automatically connected from that point.
OpenID can be significantly more user-friendly than username + password. It just isn't there yet.