This seems a bizarre one that they've never done this. But why on earth do they only offer that pointless "last 5 ratings" in all competitions, and not provide us with the players overall average rating for that season?
The "sum" calculation is clearly coded, as how else would they track the top 5 players in each league, so why don't they do this for all players in a team? Or more accurately, why don't they "show" it? As the statistic is clearly sat in cyber space somewhere.