Because we don't have the time needed to do the audit nor necessarily the access, while audotrs and regulators have both because if regulatory support and the fact that it's their full time job

The entire point of an independent audit is to throw an expert at the technologies in question at the point of the supply chain they are tasked with auditing in order to better understand it than a layperson could and make an assessment that provides the public with the benefit of their expertise. Not everyone can be an expert on everything, but experts can check each other's work and report on it publicly.