These companies/people are paid not for their general purpose skills/consulting. They are paid for trust/brand.
"""No one got fired for choosing McKinsey/BCG/Deloitte/IBM/MS etc"""
A brand's goodwill (trust and so forth) is a function of its ability to deliver success for the client. Success establishes that trust. People who run projects internally, and use consultancies or agencies, still have business requirements like any other team (whether internal or external). I can't imagine a team would be immune to scrutiny of a project failure (i.e. from shareholders) simply because they relied on a third-party. The same argument doesn't make sense if Amazon AWS suffered downtime, for instance?
I can't imagine a CTO or VP of Engineering turning around to a team that failed to deliver as a result of poor partner selection, and saying "nah, you're all good, we'll blame them." As blaming a third-party doesn't re-coup the millions in lost opportunity, shareholder backlash, press, etc.