Because they don't realise that in 200,000 years time when the descendants of humanity are looking at pictures of how we are today they will often say, " they look an awful lot like monkeys, don't they".
As regards to Tacitus, in the Annals he mentions Christians, he does not confirm the existence of an historical Jesus. There is also a possibility that this passage is a fake. It was never used as a source by early church fathers when they wrote on the persecution of Christians. As a widely read...
I think you meant have become more enlightened rather than the pejorative fallen. Because Europe was mostly a top down society and attitudes percolated down in those societies. Most educated aristocrats never really believed in god and after the enlightenment god didn't really serve any purpose...