On the spiritual level, because the world first hated Christ so it now hates Christians.
On the cultural/psychological level, I think there’s a sort of casual anti-Christian bigotry that deceives itself about being bigotry: many people have swallowed a propagandized version of history that frames Christians as powerful, oppressive, anti-science, hypocritical. And this allows individuals to think of themselves as plucky underdogs punching up at a villain class (instead of punching down at a victim class) when they insult or persecute Christians. And by framing Christians as ignorant/hypocritical, they make themselves feel intelligent/ethical for rejecting Christianity – while bonus, also freeing themselves from all those pesky ethical obligations that Christians have.
It’s a heady combination. To simultaneously get to become your own god (make up your own values according to whatever’s most convenient/comfortable in your personal culture/circumstances), while patting each other on the back that they’re MORE virtuous for it because ugh, Christians are ignorant and evil, so rejecting them and doing what we want instead is actively good, not just permissible…
Y’know. It just is what it is. People of goodwill can still find the Church and convert (I did). But there are a whole bunch of stumbling blocks, both cultural and psychological, that people can be tempted to follow astray along the way. But I mean, none of them are insurmountable. God helps those who truly seek Him, to find Him. We just can’t be surprised that the world is full of constant anti-Christian nonsense, though, in this life. It’s what God told us to expect.
I sometimes giggle to myself though, thinking of that moment when someone realizes: “… Wait. What if, in a shocking twist, the people who look so wholesome on the outside are also wholesome on the inside? What if the twist is that there’s no twist? What if the Christians aren’t secret bad guys, but are actually straightforward good guys??” I imagine many people have come to that thought fairly late in life and had their minds absolutely blown, because they’re so pre-programmed by popular culture to think that ANYONE who looks wholesome and practices Christianity on the outside, must be secretly deviant and evil and hypocritical on the inside. As you say, it’s a complete cliche at this point. It’s the default. The wholesome Christian character turns out to ALWAYS be a mask for a secret bad guy… to the point that I think for many people, it’s a genuine paradigm shift the first time they consider the possibility that Christians are actually good guys after all.