Once upon a time, barbers were surgeons. This offends modern sensibilities but there’s a logic at work here: after all, they use approximately the same tools. And at one time, physicians didn’t care for cutting people open, removing tumors, extracting teeth and so forth. Hair, teeth, tumors...whatever you’re removing, the tools and tasks are pretty much the same. Scissors and knives; blood and screaming.
So...why shouldn’t huge corporations, when they need to build a public-facing web application (read: e-commerce, extranet, CRM tool...) turn to advertising agencies? It’s all marketing more or less, right, and don’t software companies hold themselves above the squishy work of web design? The tools and tasks are pretty much the same anyway. Comps and wireframes; blood and screaming.