Ha, I'm at a worker-owned co-op conference right now asking this very question. I think it goes to lack of awareness more than anything and (untrue) perceptions about co-ops not being compatible with capitalism. While co-ops can be of any size, I personally believe the largest opportunity for co-op conversion is in the small business community, which employs nearly 50% of American workers. These businesses rely more on bootstrapping and small loans to cover startup costs and are less likely to answer to external shareholders.

Here's another great Medium article by the co-director of Start.coop about co-ops and startup capital that may better answer your question! https://medium.com/start-coop/how-to-invest-in-cooperatives-part-one-52f69350d16