Having worked with people with learning disabilities, I know that that kind of work is far more demanding and stressful than any manual work - and I've done most types of jobs in that sphere too, but the point is that pro footballers are elite athletes, expected to perform at an elite level week in and week out, at skill and athletic levels most people aren't capable of, and are paid according to market rates.
No, it isn't fair that they get paid so much, and are so cossetted , though they can soon be on the scrapheap if they don't make the grade, or lose form , or are injured, but society , and life, isn't fair, especially today. football earns millions for huge corporations, and that's why top players are paid so much, whereas essential care work, doesn't earn bring in huge amounts of money , it only costs money, and that's why it is low paid, and people who do expected to carry on working however worn out, or whatever other more serious problems, they might have, and if they can't do it, like most jobs, they are out the door.
Though, as I said, a footballer will be out the door if they can no longer produce at an elite level. Concern is only raised at 'fatigue' levels because results ultimately mean success, and success means money. A player only needs to lose half a yard because his leg muscles are a bit tired and that can mean the difference between losing or winning an important game - a game which, in the case of promotion to the Prem for example, could earn millions for owner and club, which spreads down to everyone involved with it. Players are a financial asset to football clubs, there's little sentiment involved these days.