Companies can decide to do it however they want. They don't have to conform to a platonic ideal of a bureaucracy. A company expecting to grow its market share will give it's managers more leeway than one that's under financial difficulties.

What can happen, which is why you may have seen budget constraints prevent profit-maximising hiring, is that the finance dept is operating off older information than the managers in business units (or just is crosswise with them for any reason at all). You can definitely get a situation in which managers can't get approval to do profit maximising hires, but this isn't inevitable.