- People tend to hate managers, they don't trust them.
- Flat team are faster removing the management complexity.
- There's more flexibility and more creativity around the problem tackled by the team.
The last reason is reasonable but is true applied for small unit inside the bigger organization, not the whole organization, as this article explains well. Nothing prevent to create a small unit with a specific task inside a bigger organization and to organize this unit (usually made by "star" employee) in a flat structure.
The second reason is disputable, it doesn't scale has the article suggest.
But the first reason is not addressed. And it is the real issue. I think everyone had terrible bosses and great bosses. We usually remember terrible managers as an example of the average managers (Mental association: Dilbert's Pointy Hair Boss) while we consider the great bosses we had as mentors, role models, guide. And we forget that they were also our manager. It is a form of selection bias.
There's also another issue at play, changing organizational structure, trying Flat or Holocracy is easy (I'm not saying that it's easy to do it right, it's easy to give it a try). Promoting the right person, with the right skills is difficult. And if you promote the wrong person (it happens) removing from a managerial position or moving him to a position that is a better fit is extremely difficult. So we all decide to do what is easy instead of what is difficult. Not sure I could blame someone, it is the people mindset, but doesn't mean that what is easy is also correct while what is difficult, when done right, proved to be correct.