Because they drink Cholov Stam!
From AskMoses (regarding the law of Cholov Yisrael):
This law also has spiritual reasons. Drinking unsupervised milk causes agnosticism. The milk may in fact be 100% from a kosher animal, but if no Jewish person was present to watch the milking, it has the effect of casting doubts in our core beliefs.
Boy, am I glad that *that's* cleared up. And here I thought people might become agnostics over deep intellectual or emotional issues. But no, the culprit has been my Hershey bars all the time.