Why Mid-Handicap Golfers Tend to Get Stuck?

If you've been club golfing now for a couple of years you would have met many mid handicappers who have remained such for as long as you would have known them. By mid handicappers I am referring to those who typically shoot between 10 and 22 over - they would be playing anywhere between 82 and 94 on a regulation par-72 course. Ever wondered why that is?