> It's the same reason you don't need to run a double blind study on whether seat belts work

Actually, seat belts are a weird example. After they were invented, there were more car crashes since people trusted they were protected. Without seat belts people were more cautious. They were a net positive of course, but some different situations/inventions/studies might have effects that are the opposite of what you would expect.

That’s a myth https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1540-6237....