To me it seems to come partly from inexperience with mass actions or organizing. I was definitely more of an anti-hierachy anarchist/idealist at times in the past, but when I started getting experience with protests, I saw how groups that were too focused on "democratic" decision making in the moment couldn't act as effectively on the street when responding to police or other counter-groups. Like the importance of a vanguard in coordinating the mass of people in the macro sense, it's much more effective for working on the ground quickly.

Growing up in capitalist societies I think most leftists in the west see only the way hierarchies have been used to fuck people over, and have also been subjected to anti-socialist propaganda that paints any powerful leader of a leftist movement as inevitably tyrannical. What helped me get over this bias was understanding the war aspect of the class war, that it is an actual war with an enemy that wants us dead or enslaved, and in times of conflict history has shown how the effective organization and logistics of a group is an essential key to victory.