Do you have any idea what the word "average" means? Being above average in quality means that you are better than more than half of the guys out there. The league IS broken perfectly down into below average, average, and above average. By the very definition of what "average" means, that's necessarily the case.
I get what you're talking about, but that's the difference between "decent", "good", and "bad". These are entirely qualitative terms, and are judgments of quality. They are not necessarily made relative to other players in the league. To talk about something in terms of average, though, means talking about where it falls on a distribution of other players in the sample. This is obviously difficult to quantify precisely in many cases, which is why it's difficult to talk about an "average" blocker in more than a general sense, but that doesn't change the definition of the word.
For example: back in the 80s, what would pass for an "average" Patriots season would be far BELOW what we consider average now. That's because average is defined only in relation to the accompanying datapoints.