I'm sure there are people who do it because they're just selfish assholes who don't care about anyone but themselves, but something tells me that more often than not people go to work knowingly sick with the 'rona because they literally live paycheck to paycheck and if they miss any hours they can't pay their bills.
I am fortunate that I've not had to face such financial instability in a long time, but I know for every guy like me there are 50 who can't pay the rent this month because their hours at work got cut back through no fault of their own.
It puts massive amounts of emotional stress on people and when that happens you get a lot of lashing out and seemingly irrational behavior.
There's an old saying around the 12-step program halls that goes like this: Hurt people hurt people.