[brazzy]

Your view is ridiculously myopic.

The point is not about who strictly has access to the information, it's about whose job it is to deal with it. If you started dealing with other departments asking for information and perspectives, you'd have much less time left to actually do your tasks, and you'd run into cases where they will be very reluctant or slow to give you the information because compiling it takes time and they'd rather focus on their own tasks. Or simply because they don't like you - "people problems" are also something managers have to deal with.

And it's not about "if you're planning to do something at some point, inform us now" - if the plans are concrete enough to tell you, you'll be told, and if your input is needed you will be asked, but it makes no sense if the plans still need input from 3 other departments and may change according to that or be cancelled entirely.

It's a division of labor, plain and simple. Managers have to deal with a huge amount of small tasks that involve waiting for responses from other people and acting on incomplete information. It's not easy, and you'd probably hate it if you actually tried to do it, but it's necessary for an organization to actually achieve things.