In my town, regardless of industry, warehouse workers are mostly employees of temp agencies, and usually only get hired on at a company after working there as a "temp" for over a year.
There is nothing sinister about it, these jobs have high turnover, from all causes, and generally they would have to hire multiple HR people just to manage them. The temp agencies are in a better position to manage workers who often don't last in an assignment. Maybe Joe Worker does fine the first 3 months, but then starts to have "personality conflicts." The temp agency can actually still make good use of this worker, and can reduce the overhead costs by avoiding assignments that require a lot of training. An in-house HR department isn't in that situation with these workers, so even if they spend the extra money to manage them, they just have to fire them anyways. It can be literally "impossible" to have a well-managed in-house warehouse workforce, because you can't evaluate, hire, train, and fire, workers fast enough. So you choose between being under-trained, or under-disciplined. The temp agency doesn't have to evaluate, hire, fire every time. They've abstracted out the different parts. So they can provide higher quality low-level workers than you could hire on your own. And when you want to change one out, there aren't a bunch of rules and red tape; nobody is getting hired or fired. You're simply ending an order on temp number #999999, and requesting a replacement, who will show up on time in the morning. And if it was some BS reason, or a "personality conflict," or somebody that got bored after a few months, that worker will call in every morning and probably get another assignment shortly.