Really it’s actually capitalism that supposes people are too dumb to make their own choices or know how a business is run, and thus shouldn’t have say over company choices.
Really it’s actually that businesses with that structure tend to perform better in a market economy, because no one forces businesses to be started as “dictatorships run by bosses that effectively have unilateral control over all choices of the company” other than the people starting that business themselves. You can literally start a business organized as a co-op (which by your definitions is fundamentally a socialist or communist entity) - there’s nothing preventing that from being the organizing structure. The complaint instead tends to be that no one is forcing existing successful businesses to change their structure and that a new co-op has to compete in a market where non-co-op businesses also operate.
If co-ops were a generally more effective model, you’d expect them to be more numerous and more influential. And they do alright for themselves in some spaces. For example in the US many of the biggest co-ops are agricultural.
If a theory and every attempt at real world application of a theory yield wildly different results, shouldn’t that suggest something in the theory is deeply flawed?