The reason why the obviously stupid policies of multi-culturalism exist is that they serve capital and facilitate the triumph of neo-liberalism.
Take any developed society. When discontent begins to brew, immigrants are brought into the country in massive numbers. These immigrants come from the kind of poverty that citizens of this developed society cannot even begin to imagine. For them, the status quo that the locals want to change is paradise. Immigrants see the protesting locals as spoiled, ungrateful brats.
Of course, the locals and the immigrants could start communicating, collaborating, explaining their respective positions to each other. But this is where multi-culturalism comes in.
The policies of multi-culturalism create ghettoized societies where locals and immigrants hate and fear each other. Multi-culturalism makes immigrants so unacceptable and repellent to the locals that any form of peaceful contact between them is impossible. It also precludes immigrants from integrating into the host society in a way that would be even minimally comfortable.
The result: a deeply fragmented society that envisions no possibility of uniting to oppose neo-liberalism.