The hidden back picture to this, I can only assume is they tried it and it did not work. They tried to correlate site useage metrics with subjective guinea pig tests of quality, and came away with little or no correlation. If it had worked, I am sure they would use it - they say they do not.
So they are stuck with the link authority (page rank) and content model and trying to improve that.
The biggest change of recent times is to penalize some forms of linking in order to actively penalize spam. Page Rank may help with that.