As the heading states it is the average.
As such I don’t think they are underpaid, the better players obviously get more than that and deservedly so.
An average worker gets paid equivalent to their skills and work ethics, with hard work and learning they can progress up the corporate ladder earning better money. Same goes for a football player, train hard play well and you get a longer better paid contract.
Yes I know some of you are going to comment on the average length of a football career is short compared to pretty much any other job that us "normal" people do. But even average players are given opportunities to either do some kind of degree or more specifically buy into a business that for one expands that time frame considerably or two they wouldn’t necessarily get that opportunity if they hadn’t played AFL.