“Why would they switch to human labor? “
Without unions, human labor is more flexible. You can have your driver fill in for somebody else while he’s back and waiting for the next load. The person you’re answering has a point. When human labor was free in Greek and Roman times there were no labor saving machines. The Greeks understood pumps and steam power. But a famous ancient Greek text explained that slave labor was always more cost effective than building machines. Then, as labor became more expensive labor saving machines came along. As domestic help disappeared and the lady of the house had to work along came vacuum cleansers, electric irons, inside plumbing, etc. As labor becomes more and more expensive and dangerous, employees get replaced by machines. By dangerous I mean that your employee can get a free lawyer and sue you. The more expensive (all manner of costs, not just wages) then the fewer people who will be employed.
There are now more unionized government workers than in private sector America...
And those in the private sector wonder how government became so bloated, controlling and corrupt?