It is as simple as “X is available more cheaply from a third party supplier.” But in the case of services that depend on low-skill labour like room cleaning, there are no presumed economies of scale. These companies don’t get the same work done with less labour, like utility suppliers. Instead, they pay workers less, and use complex business structures and confidentiality agreements to protect the client from the consequences (unions, lawsuits and PR damage). This isn’t capitalism realising positive-sum efficiencies; it’s just taking resources from workers and giving them to business owners.