I don't think it's that they can't change. It's that they believe they shouldn't change. They are, after all, cultural relativists who believe that no culture is better than any other, simply different. If you accept that premise, attempts to get people to assimilate are simply cultural imperialism. It seems that it never occurs to them that some aspects of culture are diametrically opposed and will result in conflict if neither culture changes.
I don't think it's that they can't change. It's that they believe they shouldn't change. They are, after all, cultural relativists who believe that no culture is better than any other, simply different. If you accept that premise, attempts to get people to assimilate are simply cultural imperialism. It seems that it never occurs to them that some aspects of culture are diametrically opposed and will result in conflict if neither culture changes.