Because the government shouldn't be paying 80% salary to the staff on min wage doing the cleaning so that the club can keep paying Mr kicks-a-ball about ££££££.
If the players cough over a fraction of what they earn, the clubs can redistribute that and keep everyone on full pay without help.
Simple, really.
And not unusual - I work for an SME. We have some staff that we will have to furlough and some that will keep on working. Before the announcement was made (and after it still) our MD, myself and our other senior managers had already agreed we would pare down what we took at payroll this month (and that's our only income - just to be clear - no dividends here!) to a minimum so that we could keep cash flow in the business to keep paying staff, whether they were working or not.
We're not alone - its actually pretty common from having talked to our contacts at various places.
I suppose the alternative would be for the clubs to furlough the players as well - after all, they're currently not working!