The other teams would spend 6 weeks doing nothing but planning activities. After the planning was done, they'd do a 12 month dev cycle. Once that was done they would throw what they have over the fence for the QA cycle.
That is the world scrum was introduced to. People already thought planning and separate and couldn't be mixed. Scrum dragged them closer to interweaving them. People resisted, saying it was reckless and unprofessional. They looked at this and thought "so the plan is we don't plan anymore? we just wing it?".
I think Agile has won enough that perhaps many specific processes meant to bridge the gap are now dragging people away from being more Agile.
The root cause is that most people are honestly not too amazing at this corporate teamwork thing.
External frameworks like Scrum can improve things 10-20%, maybe.
It's not. Scrum is a training/introductory agile framework. There is an evolution to scrum built right into the framework that transforms it into not-scrum. The framework encourages moving from the initial (and what most orgs think is the final state of scrum) "Type A" where the team work in isolated timeboxes, with downtime between for planning etc. through "Type B" with preparation(s) for upcoming sprints, and feedback from previous sprints, overlapping the current sprint, to "Type C" with complete "just-in-time process" transformation where doing any given ceremony is done on-demand. And, crucially, it encourages evolving _beyond_ this.
Type C will be familiar to anyone practicing Kanban. Or dare I say, anyone with experience with "doing agile right" will have an experience matching what Type C describes.