> which I bet are not as different as you'd expect, a lot of this is federalized

I've lived in 5 different states over the course of my life. The operating environment varies substantially. Local ordinances vary greatly. Fuel costs. Climate (vehicles in Phoenix are not exposed to rust, salt, etc while here in Michigan they are). Local labor laws, unemployment costs, insurance costs and restrictions. It's literally completely different in every state.

Businesses love to subcontract, particularly when it is outside their core competency. It's a line item on the expenses.

If unions did not exist I would posit that they would still operate this way.