Because a large number of atheists believe that even acknowledging a religion is a slippery slope into accepting its ideas as having validity.
Even describing oneself as agnostic is enough to drag people over from /r/atheism on Reddit into insisting that such a thing is impossible and you must either be religious or atheist and if somebody should insist that no, they're quite open-minded but see no hard evidence for either World view, they will be shouted down and called a liar.
Some parts of modern atheism is so toxic, it hardly a surprise that so many people wish to reject it. This is a perfect example of the kind of interaction that makes some people think atheists are just inconsiderate dickheads lacking basic manners or empathy.