Not wanting to infect your coworkers is common decency.
Joe has a bit of cough and fever and isn't feeling that great on Monday morning.
Joe wants to avoid losing money for the day.
Joe calls his boss and asks if he can work from home since he isn't feeling well.
Joe ignores his symptoms and goes to work anyway, so he doesn't miss out on his pay.
Joe gets all his work done at home while being sick with no loss of pay.
Joe infects his coworkers and everyone has to self-isolate for 2 weeks.