The EY point is interesting. I'm not sure if it's still this way, but when I worked there, a lot of care was put on audit clients as the partner(s) signing off the work had effectively unlimited liability, and could lose pretty much all their money in a worst case scenario.

Unfortunately audit work, where the company decides on their auditor, has an in-built conflict of interests. If the auditor is too harsh/rigorous, then they risk losing the audit. If they're too lax and miss something material, then they risk lawsuits and regulator attention.

Then for large companies there's further complications liket doing the audit may preclude a company from doing other (more lucrative) consultancy work, or that large companies essentially only use one of 4 companies to do their audits, which leads to people rotating through that but little effective competition.