Because the goal is to have something that finds any particular information I want, not go to every site and hope it may show up in a site search?
Because you still don't know if StackOverflow will have the result you're looking for
I've seen this in shopping results -- I'm probably going to buy from Amazon, but I like to see the top N competitors' results along side the Amazon results, so that I can easily confirm that no one is more than a buck or two cheaper than Amazon and can then feel good about my purchase.