Workers already have a lot of human capital tied up in their workplaces. If they take major financial stakes, they increase their exposure. To be sure, they reduce certain risks, such as managerial opportunism. But they become more exposed to risks outside the enterprise. A general downturn in the industry could wipe out their investment.
Diversified investors are much less exposed to such risk.
This is the reason that, when workers take ownership stakes in restructuring corporations, they frequently sell out their stakes after the restructuring is complete. That's what the Chrysler workers did after the successful bailout of the 1970s, and it's what the United workers are in the process of doing now. When the enterprise is in crisis, risk-taking is unavoidable. But once it is stabilized, workers would like to reduce risk by diversifying, which means giving up control. Another consideration is that, for many externalities, government regulation is a superior response to expanded ownership.