warehousing and many other jobs at that pay level are notoriously high turnover and not having to manage the people directly can be a good choice for many organizations.

the idea it is merely to escape benefits is not really valid. now the ACA was modified so that self insured and group insured companies must offer benefits after ninety days. the turnover prior to that can be very high so escaping that rule isn't the primary reason to contract out, its to reduce the HR costs associated with hiring.