What anarchists are really worried about is "nonconsensual hierarchies". They are definitely necessary sometimes and people will often choose to participate in a hierarchy when they see its usefulness. Anarchists have done this a lot historically, like in the Spanish civil war, there were military hierarchies among the anarchists.

The problem in most societies is that a lot of the hierarchies, particularly the very important ones for survival and day to day life, are completely forced upon us.