I see this a lot in consulting. When a new CIO (or CEO or other C level) arrives, they want to make their mark with a digital transformation intiative. This usually just means that the new C level employee is coming into a medium to large business and would like to add a bullet point to their resume and get that new shiny object everyone is talking about. Tableau, Salesforce, Data lakes, blockchain, ERP, Identity Man…
To provide a prospective as someone who works for a consulting firm like the ones you've mentioned... Hiring the "big" firms versus boutiques is a lot about a perception of risk, maintaining partnerships (procurement with new vendors is a nightmare everywhere), and leveraging experiences across other F500. For big implementations, think any ERP, our consulting teams can number 100+. Overkill? Probably, but there are few boutiques that have those kind of resources. And each of those 100 have more experience doing implementation work across a range of companies. It's a bit of a vicious cycle for boutiques where they will ultimately struggle to be competitive in these types of bids without seriously underpricing their services, which ends up meaning fewer resources or a less comprehensive scope.
As a side note, when projects go south, the company CIO isn't getting fired, but I've know many leaders (from the consulting side) getting let go for botching multi million dollar contracts.