Regarding your last paragraph, where you say that a foundation provides better insulation from founder control, it sounds to me like you answered your own question: Retention of control vs. insulation from control is precisely the distinction here.
Foundations are typically for people who don't have the option of retaining control, even if they wanted to, because they are typically close to death and in the process of structuring an inheritance. Handing over control to person X is something they see as a threat, because they assume that X will screw it up, so they'd rather make it so that no one can have control.
With social entrepreneurship like Ecosia, founders are typically still young and somewhat idealistic. They want to retain the control, because them being in control is not something they see as a threat. Rather that's what they see as the best possible mechanism for their company/cause retaining its idealistic values. (Also, they are looking for something meaningful to do with their lives).
A cynic might notice that you're kind of looking at regular narcissism vs. communal narcissism here.
If you wanted to structure your social entrepreneurship type business as a foundation which owns 100% of the shares in "your" for-profit corporation, that doesn't work as a "have your cake and eat it too" solution, because either that means that the trustees of the foundation are actually your boss and you're just a replaceable employee with replaceable employee wages or, if you try to pull any shenanigans to make it so that this is not the case, the trust loses its tax-free / "community interest" status. Trying to game this system is something that rich people are routinely trying to do. I'm not saying that some don't get away with it, but the authorities generally have a lot of tools at their disposal to fight this sort of thing.