That's because flat structures are often, or often turn into, "flat-in-name-only" structures.

I don't think the Tyranny of Structurelessness is arguing in favour of hierarchy, or against other forms of organization than hierarchy.

I don't think it's arguing against "flat" or "anarchy" style organizations either.

In essence, I think it's asking us to do whatever we're doing better, more honestly, more effectively, and less stressfully. By acknowledging, clarifying, communicating, and seeking to understand the real operating structures, what's really going on. And then to improve them, using that understanding.

An actually flat organization might be good, I don't know. I've never seen one. I've been in some that claimed to be flat, and became stressful places to work, for the same usual reasons hierarchies can be unpleasant, including incompetent bosses (not called bosses). But I've also had some pleasant experiences in flat organizations, and I prefer it that way, if it's designed and run well.

You don't need strict hierarchies, necessarily (in fact I don't believe 'traditional' hierarchies are in practice ever actually ironclad: the org chart is only ever an approximation of the real power structure). It's more that you should plan your power structure carefully (many forms are possible!), and ideally make it as transparent as possible, as pretending that you won't have one at all is merely an illusion (you will never completely succeed! Firstly because power structures are, in their full glory, ludicrously complex and ever-shifting, but also because hidden information is itself a form of power)

the argument isn't that flat organizations don't work, but that they're even more insidious than actual hierarchies. Zizek gives two great examples of the startup and the modern family. In the startup your boss is still your boss, except officially he's your friend, so you can't even hold him accountable, because there are no bosses. (political analog, traditional socialist parties, we're all comrades so better don't think anything wrong)

In the non-hierarchical family you aren't just ordered to dress up and go see grandma, you're guilt tripped into feeling bad about not seeing grandma, until you do it out of "your own volition". In the non-hierarchy you're not just supposed to be outwardly obedient but free on the inside, you're supposed to be obedient on the inside too. They work well, really well. Unlike the traditional hierarchy the tyranny is absolute because it has no borders and doesn't have to acknowledge itself.

>However the problems with traditional hierarchies are so well known

exactly, because they're visible. Valve, despite its utopian conditions, is weirdly enough, very secretive