I'll tell you - because there are no alternatives.
If you're a non-tech company (or the government), there is simply no way to hire the number of people needed for a large project. It's very, very difficult (if you aren't a software company) to build a team that's any better than what a consulting company will assemble for you. And besides, you might need them all only for a few years.
That leaves the company (or govt) dependant on an IT services vendor. Now why Accenture? Scale. If you're the government (or Coca Cola), you can't risk going to a vendor who has a 1000 employees. There would just not be enough depth if something goes wrong, and they'll lack mature Business Continuity processes. That leaves you with big IT vendors with global prescence.
Now, you could take a chance with a smaller vendor. But if anything went wrong, you're going to get fired for it. With Accenture, you are less likely to.