It's because capitalism the pejorative is distinct from capitalism the naturalistic economic theory and a lot of people actively refuse to understand this. Unless your anarchist society is truly post-scarcity, you will end up with commerce and value proxies regardless of how much you wish otherwise. And even in a material post-scarcity society, there will still be scarcity in the form of things like artistic talent, companionship, etc. If you don't want to call that capitalism, then you might as well just define capitalism as monsters under your bed.
There is no post-capitalist society besides the one focused on harm reduction. And then there is no utopia, no end goal, only an eternal struggle to combat the evils of where material scarcity and human greed intersect.