It does not prevent flaws from being discovered. It slows down the discovery of some flaws, but this also means that the people who discover them can exploit them for years if they're malicious, because flaws are equally hard to discover as part of a well-intentioned an audit as they are as part of an evil plan.
It also allows flaws that have been intentionally introduced to be hidden from sight for a long time -- and allows companies who do so to escape untarnished; since the commit logs are not available (and the source code is often only available in disassembled form), they can always claim it's a bug, not a feature.
It does not provide additional safety, it provides only an illusion of safety -- while making actual safety a lot harder to implement, for everyone.