You have your context, your manager has everyone's context. They cannot understand the details of everything, but you cannot make strategic decisions (budgeting, staffing, which projects to do at all) because you don't have the overall picture and the input frim everyone else.
The moment I start dealing with other teams/depts, I almost certainly have more context about most things than the manager "managing" me. You end up asking other folks "why X? why Y?" and get their perspective in their terms, and incorporate that in to however you're getting your stuff done.
The only thing "managers" have access to is info they choose not to share, or info they're told not to share, which creates an explicit vaccuum/silo of info, and increases the power imbalance. Most of the teams I've worked on have been this way. Yeah, sure... I don't have "the big picture" you created on a corp retreat with other dept managers... but why don't I have it? Because you've chosen to only share bits and pieces... "for your own good". Makes very little sense to me.
If I know you're planning on doing "functionality A" at some point, inform us now, because that very likely has an impact on decisions I'm making today. I'm far more likely to understand the impact on my own work and my team's work than you are, given that you don't actually know how to do the work (in most cases).
I've had a couple good manager-types over the years, but only a couple.
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.
