> Except that every incarnation of Agile that I've encountered is a rigid implementation of scrum plus sprint planning plus Jira.
This is not a problem with Agile; anything that works anywhere will lead to imitations that steal the name and attempt to extract some simple recipe from the "lessons" of that thing that worked.
> While I agree that this is against the agile manifesto, that's no excuse. This is how it ends up getting implemented in large corporate environments
No, its how something that is nothing like Agile gets implemented in large corporate environments and called Agile.
Fundamentally, this is a symptom of a broader leadership culture issue environments in the authority structure and culture has people that neither know nor care to know about the domain have authority for decision making within that domain, and its certainly beyond the power of people external to the affected organizations with an interest in particular approaches to problems in any given domain (software development or otherwise) to do much about. It is, however, a pervasive problem in large bureaucracies (not only corporate ones.)