Because auditable does not mean the same thing as audited. It is silly that people keep pushing that dead argument after Heartbleed pounded a stake through its heart. Audits are time consuming, challenging, and boring. Experience shows even critical, high profile projects hardly get any review at all by the "world", let alone actual audits by competent domain experts.

Quality verification depends on auditing and auditing depends on competent, trusted review and testing. Global transparency is not a substitute for auditing except when discussing absolute rock-bottom standards. Reviewable is significantly better than unreviewed and unreviewable, but that is the lowest possible bar. Unfortunately, software does, as a general rule, have rock-bottom auditing standards, so FOSS does provide meaningful assurance increases in many use cases. But, that is a artifact of the abysmal quality standards rather than any sort of inherent auditing advantage that FOSS provides. That is not to say that global transparency is not valuable for other reasons, but it provides no meaningful quality verification or auditing advantage in serious applications versus local transparency (to the trusted reviewer).

If you want to see how this works in practice, look at literally every other industry.