Because once again, players are human beings, even if professional, morale can tank and then performance goes through the floor. I don’t know what you gauge as best effort because one can run and output in a way the data might not refute, but the mind and the heart might not be in it and it’s not just a case of turning a switch on and off. Players slump in this way.
And when you say rationalise, it does strike as odd. Work misery is work misery. The common counter is to state how much these players earn next to the layman and that’s that as if it’s that simple and relativity doesn’t exist, meanwhile you have arguably the best manager in the world stating he’s hanging them up because of burn out and the energy it takes to fire up a club being too much essentially, he’s talking about stress, whilst being recognised as the best 1 or 2 ranked manager on the planet. Take out the human element and the fact he earns a fortune relative to the regular Joe, and he should never leave, being as top of his profession as he is.
Bottom line is morale might well be even more important in their field than ‘real life’. Given the difference even a few percentage dropping can constitute.