As someone who started and still works in a co-op, it's because it's hard. Banks don't understand worker coops and won't lend money to you without a real person to attach the risk to, which means founders have to take an enormous risk which it can be hard to compensate them for. The legal structure isn't common so you are limited in the lawyers who can set one up for you. Others have mentioned the cost problems - I started a software dev coop so we didn't have a large capital outlay but it did cost nearly 10k just in setup costs.

It took a lot of work to get to where we are, with little supporting resources. In contrast, I started an LLC in half an hour and $150 registration fee to the government. So no, it not just "what people choose".