It is not that there cannot be multiple points of failure; it is that it takes just one point of failure to bring the entire system down. As long as there are dependant and moving parts failure will be inherent in the system; it cannot be "inspected/tested" away. Adding redundancy does not remove failure it merely transforms/postpones it. Failure is the "natural" state; we intuitively understand this through the concept of entropy always increasing (or more colloquially; "everything is going to shit") unless we do something about it i.e. expend energy and do work.
Therefore there has to be a single cause of failure, because all else failing the CEO has mucked up. And practically speaking there is probably a root cause somewhere closer to the problem.
"There are multiple causes of failure" style arguments are technically true, but also the sort of argument that people make when they aren't going to do anything to improve. There are many things that could change. There aren't many reasons why there was a failure.
> Why was the chain manufactured with a weak link?
I don't think it is feasible to manufacture a chain without a weakest link. The materials science requirements to produce truly identical links are extreme.
The people who truly improve systems are, in my experience, the ones that do consider multiple causes of failure. The ones who latch onto narrow chains tend to fix symptoms and then think they're done.