Because that has a specific meaning, especially with regard to insurance. (And there are a lot of mutual insurance companies out there that are genuinely, 100 percent member-owned.)
But you mention venture capital and a rate of return for your capital partners.
So how is Goodcover what you say it is? Sounds like a for-profit company to me.
Saying you operate as a co-op when you're actually a for-profit is like saying you operate as a non-profit when you're actually a for-profit.
Ultimately, if you have outside investors, you've gotta make them money. If that's true, then suggesting you're operating as a cooperative is misleading at best.
EDIT: I looked at the legal notices section of your site, and this ain't a cooperative in any shape or form.
* "Goodcover takes a fixed fee - currently 20% of premiums."
* "Whether a Dividend can be paid is up to the discretion of Goodcover’s board."
This is absolutely a venture-backed, for-profit company that is describing itself as a "member driven cooperative." I would not be surprised if legal issues arise because of this misrepresentation, because California treats cooperatives as a distinct legal structure. (For instance, it's illegal to have the word "Cooperative" in your business name if you're not incorporated as a co-op.)