Presumably such a entity should also provide centralized arrangement of benefits, payroll, legal documents and other issues. An extreme form would be to have this management company run resource companies that make all hires on pass-through basis and submit a single consolidated bill to the main company each month.
Why wouldn't this work? I can think of only three reasons though I'm not convinced any of them are true: Insufficient supply of sufficiently homogenous companies (e.g. software startups); sufficient diversity of preferences for customization, particularly with regard to location to create insufficient density; or fear of poaching due to close proximity to other companies that require the same skillset