Reading comments on Hacker News, I saw a thread which began with "The biggest problem is the like button." In talking about the rise of social media contributing to a decline in "blogging," this seemed a reasonable idea.
The top response to that was "The biggest problem is that you can make money from the internet." Which also seemed reasonable.
The top response to *that* was "I would say the biggest problem is advertising." Which again seemed like a reasonable thought.
Maybe one issue is that humans tend to the superlative, where every issue is "the biggest problem" rather that "one issue."