Like a lumber mill that buys logs from a logging company? Who subcontracts to individual fallers? Who buy their chainsaws from a chainsaw company, their fuel from a petrochemical company? No business isn't entirely dependent on third parties. That's what it means to exist in a healthy economy.
A "farm-to-table" megacorp that owns everything from the source of every raw material through to the finished product is a nightmare, an entity that needs to be reigned in and broken up. Capitalism is a competitive game, but a game that can never be allowed to end.