I’m guessing you either don’t live in America or have never had to work a job like fast food to get by.

If they have sick days they probably either can’t afford the time off or get pressured in to not taking it by their boss/family/social circle/society

And just so we’re clear, sick days aren’t necessarily paid sick days. They can also be days you just don’t get in trouble for missing*

*May require a doctor’s note