A lot of large companies like to have standardized policies across the company so they can shuffle people where ever they need and everything is the same. Union

A lot of large companies like to have standardized policies across the company so they can shuffle people where ever they need and everything is the same. Unions are usually location by location and often only cover a specific classification of the employees in a location (manufacturing, warehouse, maintenance, sanitation, and/or truck drivers), and each union will bargain on its own so you can have a single location running with multiple unique union contracts for each classification of employee. This is a PITA to manage for big slow corporations who would need teams of lawyers and HR people working thru and negotiating each contract individually and getting approvals from above.