It depends on how you define "hierarchy," and that ambiguity is part of why some anarchists say "unjustified hierarchy," because some people say that having any sort of authority on account of having a managerial role in an organization is "hierarchical," to which those anarchists say "sure, but that's not what we're fighting against." I think this is a fair approach.

I expect if you get a lot of responses, then many of them are going to talk about rebelling against bedtime, and while there's surely an element of that in some cases, I think there are a few more productive answers:

The other obligatory answer is going to be the portrayal of basically every existing Marxist project that even momentarily succeeded in controlling a country, where there was a revolt supported by many people who wanted a better society but something something bureaucracy and then it was basically fascism. If you believe these narratives, in which these popular movements where probably the vast majority of people were fighting for liberation and forged their own chains due to those sneaky communists, it makes sense to look at the structure of those insurrections (as-reported) and come to the conclusion that centralization is evil (because this question is inevitably very tied to the "decentralization" fetish) and we need to have prefigurative communes or whatever. If you accept the aforementioned historical narratives and also believe that prefigurative communalism is structurally unable to topple capitalism, then it's very hard to avoid the conclusion that humanity is doomed, so what is there to do but hope communes can do it?

Lastly, I think it's worth mentioning that all of these conversations are inevitably going to be very informed by liberal philosophy (as is Marxism, I don't mean this as an attack), where there is a long tradition thanks to natural rights discourse of viewing governments as inherently unjust because they are contrary to human freedom (this is heavily slanted toward bourgeois freedom in our case, but I did say liberal philosophy), so it's understandable how there would come to be a fetishization of structures that are unlike what we recognize as governments and let everyone do what they like and they'll spontaneously (or with a bit of encouragement) do the right thing and that's the only way to have a truly just organization.

And if they are defeated, then that just means they were too good for our sinful world, so it's no mark against them. (Also even if they didn't actually organize that way, we can just pretend that they did because history only matters when it gives us cool stories or lets us own the tankies.)

Oh yeah, and there have been some historical communalist movements that did cool things, like the Communards, but it's also important to note that the Communards got obliterated.

I apologize because I'm kind of struggling to articulate myself properly because I'm a bit sleep-deprived at the moment, but hopefully I at least gestured at something useful.