Why would they all be working on the same thing? If you've got 20K+ employees, 100+ developers isn't all that many. Hundreds of departments across dozens of businesses leads to a few dozen projects going on at once. Add in the maintenance of apps that are often 5-10 years old taking up a large chunk of those resources, and 100 developers isn't really all that many.

BFEA-wise, the technical challenges revolve around the intersections between the various projects - message queues, batch servers, databases, storage (and more storage). Proponents of independent teams tend to dismiss these challenges, resulting in an overall environment that is more expensive to implement and manage. (for example, sure, that obscure OSS project may be _perfect_ for the task, until the one guy that knows it leaves, and you have to pay someone top-dollar to ramp up on it)

To be fair, there's usually a lot of room for improvement, efficiency-wise. To continue to be fair, I don't know that the success rate of small dev shops is any better or worse than the large: there are more failures than successes, and many more "good enough" situations.