Mostly because most people have lots of options for employment and they typically don’t take the low options. In a place with nationalised healthcare you would still be paying for it through taxes paid by the labourer but I guess the system would be a bit more progressive than in the US so lower-skill labour would demand a smaller premium for healthcare. Though I think there’s probably more than just labour costs in the US. Eg maybe it is expensive to get the right permits to install fibre optic cables.