Because in a large number of cases, people are conflating economic situation with culture or race. And many of these so-called cultural or racial components disappear in the "home" country when you look at people in the middle class and above. Just like violent crime rates drop massively for those same races and cultures in the country they have immigrated to when they gain economic independence.

It's quite possible that high levels of immigration of unskilled or otherwise unemployable people is a bad thing for a country that shouldn't be allowed. That is a very different thing than acting like the reason it is a bad idea is because of their culture or race.

Of course there is one, but what exactly is "immigrant culture"?

That is my problem with the whole thing, all just gets thrown into one bag "immigrants". The women assaulting islamist gets the same label as the homosexual atheist fleeing where they are in charge. (And never mind that the bible contains mostly the same bad passages as the quoran and I never done the numbers, but I assume if we count and plot "muslims killed by non non muslims vs "non muslims killed by muslims" - that number would not match the felt reality)

Acknowledging that contradicts liberal ideology, which is the closest thing most people have to religion in modern western society.

It's not merely a bit controversial, it's heresy

Why is it so hard to admit that there is a cultural (and, by extension, racial) component?

What does "by extension" mean? Culture is voluntary. You can (and should) walk away from a flawed culture. Race is an unsheddable fixed attribute.

Meanwhile, both culture and race see more variation within groups than they do between groups, making discrimination a losing proposition even aside from its inherent immorality.