As a European who lived in the USA for 27 years and spent considerable time criss-crossing the Bible Belt as well as living in the San Francisco Bay Area, it seemed to me that the US anxiety about atheism was driven by two main factors. One, as you correctly point out, is the high level of religionism among less educated and less informed people. The less one knows about reality the easier it is to believe in invisible magical gods and goblins.
But the second factor is also important. The human brain is hardwired to seek patterns, and the simpler the pattern the less effort required to assimilate it. All mythologies are intrinsically simplistic: mummy and daddy gods we must placate and obey otherwise we’ll be spanked and put on the naughty step. If we’re good, we’ll eventually get a treat. Meanwhile reality is stochastic, incredibly complex, and not entirely amenable to comprehension by a small-brained ape species.
Not surprisingly, by noting the intrinsic implausibility of myth, atheists cause alarm in others by showing the hollowness of their core beliefs and thereby bringing huge doubt into every aspect of their existence. It’s much easier to be angry at atheists (and by extension at reality) than to attempt to grapple with the sophisticated perspective required to leave infantile stories behind.