They are indeed rare, although I very much hope they will become more common. Co-ops aren't a radical new idea, in fact they are one of the oldest company structures we have.
I think they have become rare because the industrial economy of the last century was really optimised for command & control hierarchical organisations, due to needing to scale in an environment of limited communications technology. Now we've moved into an era of much easier peer-to-peer coordination at scale, and I think we'll see different business structures emerging. We already see this trend in how social and political movements are being reshaped.
The other big reason is the capital instruments we use to fund most startups. The VC model calls for a certain approach to ownership and control, based on financial contributions taking precedence over labor contributions. We've really only been able to be a co-op by finding alternative approaches to investment. First we did a big crowdfunding campaign, then we raised a round of impact investment that allowed control to stay with cooperative worker-members[0]. Not going the usual route with capital has presented challenges, namely taking longer to find the right model, but it's been worth it to keep our co-op structure and social mission at the core.
