Ok I thought you might like to know this /r/antiwork
So a trucker isn't allowed to work over a certain amount of hours. Either driving a truck or non driving job.
So a employer can hire 3-4 truckers when 2 are enough. Give each trucker barely any hours and save money.
The truckers can't get a second job or any other job because of limitations on time working.
So it's a glass celling.
DOT on duty rules