They're offering interest on roughly a dozen hand-picked markets, according to their documentation. (I wasn't aware of that, so I stand corrected on the general assertion that they never do.)
> That doesn't mean there aren't other explanations.
Why do you need other explanations, when the observed probability can be precisely and fully explained by opportunity cost?
I don't have to "need" other explanations in order for them to exist. The current price does happen to accurately reflect what the risk free rate would imply. But look at the graph history: it hovered around 1% for a large chunk of December.
> That doesn't mean there aren't other explanations.
Why do you need other explanations, when the observed probability can be precisely and fully explained by opportunity cost?