The real answer is that for the web to continue as it is, no such system must exist. If you require a single authentication, the web stops being a loosely couple system and becomes dependent on a single entity.
I believe that part of that design is what's so confusing to non-technical users. If somebody were to tell them that 'the box in your basement' could be used to verify access to their banking website, you'd completely lose them. Granted, its an implementation they'd likely never encounter, but the fact that its possible just contributes to the noise around OpenID.
If you were to tell people in 1985 that they would be able to see their credit card balance on an LCD of a mobile phone while jogging, you'd completely lose them too.
