It’s possible because you are just ignorant of their brand and the facts. Again, it’s fair to say he is not THE biggest star at ESPN, but the idea that he is not even in the discussion just demonstrates your ignorance on then matter. And there is nothing particularly wrong with that. We all have blind spots, and have been wrong before. However, your stubborn insistence on the matter is pretty puzzling given the facts and arguments laid out before you.
Someone already eloquently answered this question, but even if I accepted your premise, Bill Simmons is still ahead of most people at ESPN. Bill Simmons gets 1.42mm results on Google. Chris Berman gets 400k. Mike/Michael Wilbon gets about 400k total, Tony Kornheiser gets 309k, Mike Tirico gets 145k, Mike Golic gets 263k, and Mike Greenberg gets 326k. None are even CLOSE to Simmons. Even Erin Andrews only has 846k results. Unless you want to suggest a better way to measure popularity beyond, “have I personally ever heard of it”, then I think we should trust Google.