Agile requires just in time requirements and priority setting, coupled with the ability to make small changes and iterate.
If you have a "this feature needs to be delivered by X date" type of corporate culture, then you have to make a commitment, and because dates are almost always tight, you need to be as efficient as possible to achieve that goal.
So basically you have the software teams wanting to work in two week sprints, and you have execs needing new thing for contract launch in X weeks, and guess who wins? It's not the software developer, so the agile becomes toothless, because you're not learning, or iterating. You're just pushing against a gantt chart the whole way a sprint at a time, and might not even be able to release beta versions and get feedback, because that takes up valuable time.
I'm not as cynical as OP, but there is a really big push/pull that happens, and it can turn into a toxic environment if forces too far outside the product/development cycle dictate priorities (like sales, or execs).
I don't agree that Agile requires JIT requirements but benefits from and thus promotes them as the best way to deliver requirements for a feature.
Under waterfall, a document is written by an analyst that describes a problem to be fixed. Six months later, the task is picked up but legal regulations or market conditions or even the rest of the software has changed. However, the Waterfall requirements are the requirements and that's what gets implemented. Best-case scenario is that the business analyst that wrote the original spec is still employed and has been updating the requirements as conditions change.
What we've done at successful agile shops I've been a part of is quarterly planning that collects at a high-level the current requests/needs of the business, prioritizes them, they get a t-shirt size to determine if all of the requests/needs can be met during the quarter and they fall off by priority until you're left with a manageable high-level plan of work for the quarter. Then those high-level requests are decomposed into epics and stories which are specced out and estimated, etc.
It works very well but it requires that people work collaboratively from the executives and business stake-holders to the technical leadership and individual engineers.
