It's a fair point that any easy-to-find bugs in large, time-tested contracts will have been found. Any medium- or hard-to-find bugs also will probably have been found. But there might still be a very-hard-to-find bug lurking; and given the value of finding such a bug, people might look hard enough to find it.
In other words, the same scale that ensures there is no low-hanging fruit, also provides the incentive to pick high-hanging fruit.
> I have a lot more trust in that kind of product than in a product where contracts are subject to interpretations by humans which may or may not be reliable.
This is a valid concern, but it's also extremely well-understood at this point; we have centuries of global-scale experience with traditional financial and legal systems. They're certainly not flawless, but it's rare for gigantic new flaws to emerge. An important point is the existence of mechanisms for rolling back bad transactions and challenging / appealing flawed decisions.