There is no simple answer to the question "why are there so many bad managers?" One of the reasons is imitation. We learn from other managers, who may or may not be good. Because we have no frame of reference, we simply assume that if they have been a manager for awhile, they must know what they are doing.
Don't behave a certain way because other managers do it. Less than 20% of managers are considered great, so in all likelihood, you are following the behaviors of an average or bad manager.
I should clarify that I don't think managers intend to be bad and spread bad habits. It may be due to simply lack of knowledge or it could be that things have changed. The skills that made a manager great in 1990 are radically different than the skills needed in 2021.
Add to this the fact that many managers do not get any training or feedback on how to be great, and you have an avalanche of bad managers who are teaching future managers all the bad behaviors.
This is why you need to develop your skills as a manager. Learn what is good and bad behavior from experts, not simply other managers.