Agile is (maybe a bit oversimplified) about releasing as often often as possible.

Delivering sooner, not faster.

To do this, decision makers must be close to the team, and ideally a part of the team. Either managers are part of the team, or the team a given the authority to make product decisions.

Full time managers lose power and influence either way, and I think this is the real reason Big Corp never really bought into it except on paper.