Because they drank the evil corporate Kool aid years ago; it's cheaper and easier just to misappropriate someone else's service.
> Mozilla also offers a white-label version of Mullvad VPN, while subtly implying that they operate the VPN themselves: “A Virtual Private Network from the makers of Firefox.”
Maybe 'misappropriate' is a bit of a stretch, and certainly the rest of the language in the above comment is tiresomely partisan, but it's not hard to see what they meant.