Because some (and I stress, some) Leafs fans are incredibly insecure. It's why threads like 'How good is Larkin?', like so many countless threads on here that have absolutely nothing to do with the Leafs, inevitably devolve into d***-measuring contests.
Larkin's having an amazing season. He's a beast in all zones and he's never stopped improving so no reason to think he's done now. So to answer the question, he's a clear 1C. And a damn good one at that. When comparing to players like Tavares or whoever else, it all comes down to priorities. What type of player do you consider to be the best. Larkin is forming into the Bergeron mold, an elite 200 ft player. He may not be as elite as the Tavares types in the offensive zone, but the question is whether the defensive aspect overcomes that offensive deficit. I think it does. Because Larkin is quickly developing the ability to shut down other teams' top lines. He's done it to Colorado, Tampa, and interestingly, Toronto, this season.