Scarcity of resources and the means to allocate them as well as the existence of fundamentally undesirable labor.
Even if we were to assume all people were equally competent, valuable, and willing to work: resources must make it from the production line to the consumer.
If there is a limited number of resources (which there always will be) there must be a system in place where people handle who gets what. Those in charge of such a system have more power than those who aren't. That's hierarchy.
If we somehow did have unlimited (or sufficient surplus) of all resources then there still must be a way to get it from point A to point B. Those in charge of such a system have more power than those who aren't. That's hierarchy.
Even if we somehow got Star Trek replicators and the ability to beam products effortlessly to whoever wants them, there will still be tasks nobody wants to do but must be done. Either the people doing this labor are stuck in an inherently less desirable position than others, resulting in hierarchy, or they must be rewarded in equal measures for their efforts, resulting in hierarchy.
I guess we could have economic equality if we had unlimited resources and no need for logistics or labor. But then some people are just gonna be more "likable" than others or more attractive which is still a hierarchy.