To me it's the uncertainty that scares people. People feel safe obeying certain social constructs. Something about crossing that line, and "going to the dark side" terrified those who have been forbidden to even question their beliefs. It's sort of similar to when I first became more liberated in other ways that were considered taboo. The common person is a conformist who is scared to death of being condemned by others.

Because Atheists are not organised. They don't have people that protect them as a whole from vilification.

Small minorities are not seen as threats by the majority. Larger minorities are.

According to the link 75% of the US are identified as religious in 2017. 25% are other. IMO the numbers answer the question.

[en.wikipedia.org]

Because evangelicals are big mouths who never shut up.

Whenever you judge or doubt something that someone else bases their entire existence on, it's bound to cause some friction.

I know numerous non-believers right here in this rural neighborhood. They don’t describe themselves as atheists, and neither do I. When you say you are an atheist it sounds as though you are identifying that way—that you are first an atheist and only secondly a human being, a good neighbor and friend. It’s might be seen as an in-your-face kind of thing, as elitist, as though your opinion is the only reasonable one. When you say you are an atheist you are saying something negative. It would be better received if you described what it is that you DO believe.

There are a few who say they are Christians, but generally not in an obtrusive way. I think maybe if you just walked up and said that you were a Christian you’d be shunned by half the population. We’re just not that interested in your religious opinions either way.

Theist's dogma requires that non-believers be reviled, ostracised, even killed. Theists just do what their authority figures tell them to do.

Most theists unconsciously realize that their belief is indefensible. The existence of atheists aggravates their unease. Instead of living next door to an atheist, most Christians would prefer a follower of Islam as a neighbour. This is despite the increased likelihood of violence and general non-civility intrinsic to followers of "the prophet".

It is a threat to many of them because it makes them recognize their own doubt.

We need to improve our public image. Corporations use PR firms and fund an advertising campaign. We don't have for an ad campaign, so the only way we can improve our image is with grass roots "good will" programs. That takes a plan and volunteers. We aren't organized.

The vast majority of believers in the US are one form of christianity or another, I suspect most of them don't even know the correct definition of atheism.

no protection in number. think about ants and mosquitos

Perhaps we need to stop eating babies? LOL. I'm a social worker, a profession filled with caring, compassionate, helpful people (not doing it for the money!). So often times, the reaction to me "outing" myself is something like "you SEEM so nice" or "but you have moral values!". There is this misconception that we reject religion the avoid their rules. They don't realize that most agree with the basic human values regarding honesty and not harming others.

It deeply depends on where you live.....

We represent the existence of what is for them,  an unconscionable horrifying spector, that humankind is an evolved species, mortal, not of any religious deity's creation and that promised eternal afterlife. They are indoctrinated from birth that you cannot be a good, ethical or moral person with any life value or worth if you do not believe as they do.  This irregardless of the ever growing mountain of knowledge and evidence that refutes the very basic fundamental theories and beliefs of their religious dogma.  Top this off by their non-stop efforts force teaching bible based creationism or intelligent design in schools and to diminish evolution based sciences.

thou shalt not question the bluff.

In some countries people identify with a religion for cultural reasons rather than any actual belief in god or the dogma of that flavour of religion. For example in Ireland people brought up Catholic often state that they are Catholic as opposed to their old enemy Protestants.
It's a similar situation between Sunni and Shia Moslems...
There is of course an implicit irony that these tribal conflicts are often fought and spearheaded by individuals from both sides..with little or no belief in anything except killing or gaining dominance over their enemies..

I might be wrong but some people who believe in God are in that no religion category. They don't identify with a particular religion.

I believe we are looked upon negatively because so little is known about us as people. Even to those without religion we have been protrayed as bad people or people lacking any morals.How to reverse this image is a challenge.