"If it doesn't get me sick, why care?" That's why.
Just because it doesn't make you sick tracking germs/filth around can get other people sick.
And just because you're fine with shit and piss (yours or others) on your hands doesn't mean other people are.
You don't wanna wash your hands at home fine - but "refusing" to wash hands in a public setting is a shitty move.
If you're going to the trouble to reheat fast food fries with a fryer you might as well just make your own.