Because they don't think of themselves as bad people, and they are trying to devote themselves to something larger than themselves. So when someone else handwaves the overarching notion of faith, and paint it as a dynamic that causes more harm than good. That disvalues an establishment that for followers becomes equivilant with their identity. Theists seek to become in covanant wtih God, they seek to be part of something bigger.
So for many disagreeing in even a mannerful tact will still come accross as an underhanded disparagment of them as individuals. Never mind there is also the facet or self-doubt that is often present with people of faith, and temptation is seen as being the work of earthly forces. So for some there is also the feeling of a true evil coming from dissenters and non-believers.
If you truly don't believe, I think you owe it to yourself to at least understand more deeply the dynamics that incur the these responses. Because there are many just kind, moderate, theists that will really feel attacked and pressed against the wall if you take a hard stand in disbelief. Though they usually get excited if you claim you're merely agnostic, because that is a chance for them to share proudly their belief with you and not be falling on their back foot.
