Because smart contracts are (often) contractual obligations on real world things, they only hold as much power as the apparatus of coercion (usually the State) will allow them to hold. That is, you must trust the political authority first and foremost before you trust the contract. This is very different to bitcoin, which operates purely in the digital realm, where you can trust the ownership of the btc without requiring trust of the political authority. So bitcoin solves a trust problem and this makes the less efficient distributed architecture worthwhile (it would be much cheaper and far faster to operate a digital currency in a centralized way). But if you have to trust the political authority for digital contracts on physical goods, what is the point of the extra cost? I'm dubious there is any real benefit.