Because we dont want player prices to be too high either. There are good free agents who meet the criteria and get sold for the 10x salary or more. Bad ones wont get sold and retire. Free agents also take money out of the game which is important.
That's called market regulation = not free market.\
"Because we dont want player prices to be too high either. " - What does that mean? What is too high? Who doesn't want that?
"Free agents also take money out of the game which is important." - Why? People in this game don't buy in game money for real money. If they reach financial peek to quickly there are many other ways to regulate that. Fixed costs, then tickets sold, injuries I could go on and on.
Let me tell you what you are doing here.
Most of the quitters are relatively young mangers hence most of the free agents are just semi good.
With them on the market you are keeping 1 -3 seasons old managers in check. They cannot get their investment in training young talents back, because value of their trainees in depreciated by free agents and as someone already mentioned training is not feasible. This way gap between old managers and new ones is just getting bigger.
The above effect is obvious, There is hidden effect too. "The addiction" to game is largely created with Tamagochi syndrome. Taking care about something and watching it grow develops addiction like nothing else. That's completely pushed aside. Largest success would be watching own draftee to do well in your leagues. That's currently pointless because it is not feasible.