Because failures like these are rare every few years and Accenture does several hundred projects of this size a year. If you need a tech consultancy for any project above 100 people they really are one of the first choices, if you upgrade SAP or any system touched by more than a few thousand people Accenture is likely on the top of the Head of IT's mind, and that's if they can afford them.
Once a company reaches a sufficient size, they need someone like Accenture, its really as simple as that. Every FAANG or F500, really none of them do it themselves. Why would they do use any of their own expensive engineers on their back-end systems. The value is in outsourcing to someone with economies of scale for several months instead of hiring FTEs or reallocating existing staff for an x month project every 5 years.
Once a company reaches a sufficient size, they need someone like Accenture, its really as simple as that. Every FAANG or F500, really none of them do it themselves. Why would they do use any of their own expensive engineers on their back-end systems. The value is in outsourcing to someone with economies of scale for several months instead of hiring FTEs or reallocating existing staff for an x month project every 5 years.