Perhaps you are right that good project managers are not too common, however, I find it easier to reason with one single manager than to reason with an entire team who has defined themselves as a "SCRUM team". In my experience the dogmatic nature of SCRUM means that teams take on an almost religious attitude towards it. A team like that is far more difficult to reason with than a single bad manager and disagreemnts often end with the conclusion "Yes but we do SCRUM and SCRUM says to do this.".
SCRUM contains some good princeples which I agree with like iterative releases and attempting to break down stories into manageable chunks, but the reality is that SCRUM is more than just a set of princeples to take on board, otherwise it would not be a multi-million dollar industry for SCRUM consultants and such.
You are right though, SCRUM has its use in preventing bad managers and bad teams from doing bad things. However at the same time I think it hinders good managers and good teams from doing good work.