You think if, say tomorrow, all 1.4 billion citizens in China were teleported into the U.S., that there would now be 1.7 billion Americans, indistinguishable from the Americans that came before? Or, would it simply be 1.4 billion Chinese, alongside ~300 million Americans (if we're being generous...)?
Demographics and culture are obviously intrinsically tied, each individual migrant holds beliefs and values that have been forged and shaped over the span of the lives that they have lived in their country of origin. This is, and has always, been the case, its just more of a hot-button issue recently because:
A: We are currently getting an overwhelming amount of migration to stave off our own demographic issues, so the strain begins to have a more noticeable impact,
and B: Many of the migrants (from abroad) are coming from countries with cultures intrinsically incompatible with values Westerners have come to expect in their own countries (See: Sharia Law as contrasted with Women's Suffrage), so they stick out somewhat like a sore thumb when advocating to enact such policies and practicing such cultures.