The people who have access to my private repos may or may not be benevolent, but they are certainly under contract, and that's the relationship that Github is helping me manage.
The contract says that the codebase is mine, and the contractor doesn't have rights to use it for their own purpose. The Github private repo rules just make that real in a practical sense. It means the default way of working comports with what the contract says. It makes it easy for everyone to do the right thing, and harder to do the wrong thing.
Sure, a rogue contractor could re-upload my codebase to a new repo in their account later, but that is a pretty obvious violation of the contract. If I found out, I'd feel pretty confident suing them (if it came to that). There's nothing they can point to that says I authorized them to do that.
In contrast, if the default was to leave an official fork of my code in their Github account forever, then it becomes easier for them to do the wrong thing, and harder for me to say I didn't want them to do it. Some future employee could even do the wrong thing accidentally.
To reiterate, Github private repos help me manage my relationships. They don't enforce the terms of my relationships.