I would assume it's because a good few of them run like raped ape's.
There are very few low 13 street cars out there, some of the LS1's can do it.
I've seen stories of the LS1's taking Ferrari F355's top end

before. There is no way that happens unless the driver of that Ferrari is clueless. Anyway, bottom line is there just aren't too many other street cars that will usually give them a run for their money.
There still are a few though, and they are not unbeatable. I find the LS1 owners in general to be the cockiest drivers out there by far, just makes it more fun to see the expression of disbelief on their face when you beat them.