None of these existed before the Kickstarter though and I've tried about 10 different ones in Hong Kong and they where all complete shit. (most "work" but don't fit properly and the magnets are too loose or too strong)
I don't know for sure, but I would think that there is likely some kind of difficulty in sending high-speed signalling through a magentic field like that. I think there was a reason Apple only ever used magsafe for power.