Who's to blame when someone gets sick and I have to fill in for them at their jobs?

No-one is to blame. Virtually everyone gets sick sometime. It will be a miracle if YOU never do.  One day someone will, almost certainly, have to fill in for you.

I'm not sure there is anyone to blame.
I just work their shifts and put the overtime earnings in the bank.

You, because you agreed to work their shift

Maybe they wish they could work to get paid? Why is there someone to blame? Life stinks sometimes.

leftists say Donald Trump is to blame for everything wrong.