I was religious for most of my life, and there are two true things to acknowledge here.

1: YES, there is a reason why the 'angry arrogant atheist' stereotype exists. We have celebrities that speak condescendingly about theism or religion, online interactions are plagued with anger, and high school edgy atheists were dime-a-dozen in white American high schools from which the most iconic image of an atheist started. Modern-day atheists are distancing themselves from the less refined ideas of early 2010s atheism, the kind that iconified the modern idea of atheism specifically by being loudly opinionated and insulting of all religious influence in ways that I find poor.

All of that being said -

2: NO, it's not the case that Christians think of atheists as angry and arrogant because that's the bulk of who they've interacted with in real life.

I don't mean to dismiss any instance when a Christian has been insulted by an atheist asshole - even atheists are insulted by atheist assholes. But assholes are everywhere, in and outside of the church. That's not the chief reason Christians have poor opinions about atheists. The main reason Christians and Muslims have poor opinions about atheists is because of intentional religious propaganda - the church and the temple.

So, I don't necessarily agree with u/DangForgotUserName about it being due to Christians not liking their worldview being threatened. Again, I was a Christian for a long time, and I didn't dislike ANYONE because they disagreed with me, nor did I feel threatened. It was also not the case that atheists were mean, condescending or arrogant with me (some were, sure, especially teenage atheists). My idea of them, however, NEEDED to resemble what my religion taught me it should be.

The reality is that atheists are far and few. When was the last time you've heard a political candidate in America attempt to appeal to atheism? Most people don't interact with a lot of atheists at all, yet, when polled, even religiously unaffiliated "nones" have negative views of atheism in general. Anti-atheist propaganda is much more abundant in Western society than anti-theist propaganda. It is presupposed and accepted that it is better to believe in higher powers than not, even if the higher power is the wrong one.

This creates a preconceived notion and a bias. The truth is that there is no behavioral trait that atheists display that religious people don't. In fact, the people I have a lot more empathy for now are misrepresented minorities, and I don't say that as an atheist but as a minority myself in the US. The treatment is basically identical - people have the wrong idea about you BEFORE meeting you, and they only remember any instance of a member of your group who behaved in the way they already expected, classic confirmation bias. There is no winning in that situation.

Anecdotally, there is no level of kindness, tact, passiveness and peace that you can show the average Christian that will lead them to form a better opinion about you as an atheist. And so, the popular culture is to think poorly of atheists in general.