I imagine the discount on a lopsided game is because the teams stop really playing. After 3 goals or so it's generally a done deal. So you start to see players pulled, playing very differently (conservatively).
(Although the league sometimes doesn't work like that either - I remember listening to Spurs vs Man Utd when Man Utd went in at half-time 3-0 down and ended up winning 5-3 in what is possibly the most epic half of football I've heard in 30+ years.)
Also, goal difference is important in leagues, so teams tend to play to the end even if the gap is large. Though I think this could be improved by changing to 2 points for a win, with a bonus point for scoring 3+ goals.