I've been thinking and I might know why a lot of us don't find him funny:
He's a good looking straight white male with the general and majority view on life. He has nothing going against him that he can make fun of, and yes you can be funny even if you're like that but let me explain.
Dane Cook doesn't bring any funny new perspective to things, he rambles on about something we all know too well in a way that it being brought up doesn't register any excitement or any realization, its just a retelling of the same old thing so therefor its boring. Its funny to hear a black guy talk about it not being easy to get a cab in new york city and bring up creative ideas about how he's being back in funny ways, but it still relies on the execution. Dane's concepts are the norm and are general knowledge or thought, so he needs to be an excellent comedian to twist it in a way where it seems funny, so these concepts rely a lot more on execution than others, and since Dane lacks any real comedic ability he tries to make up for it with his spastick movements and surprise physical comedy. He doesn't have any real comedic timing or the ability to deliver a punchline.
But to his fans, he's more or less speaking the truth, which they find funny just because it may agree with their world view, and since this is the majority way of looking at things that might be another reason why he has a big audience.
There's more to it I'm sure, but I'm not really in the mood to watch dane cook videos and start writing about them.