There are different levels of culture. The "happy nice version" of multiculturalism involves surface-displays of culture - different groups of people have different kinds of food, different styles of dress, different accents/languages, different skin tones, but are otherwise nice friendly liberal people who believe in the core values of Western culture, such as the rule of law, equality before the law, relatively high trust in civic institutions, etc.
Except, there are literally billions of people who don't believe in those things. There is not "magic dirt" in the USA or "The West" that gives us those things - it's a function of the people who live there. If you were to import millions upon millions of people who don't share the values, while simultaneously teaching the youngest generation that white people are responsible for all the evils of the world, and encouraging minorities to nurse racial grievances, you're going to end up with problems. But it's impossible to talk about because you have unhinged racists on one side, and people who are prepared to see unhinged racists everywhere on the other.