Because it decreases social trust, the bedrock of a functioning society.

Probably if one is a nativist, one has in their mind that "migration" means someone coming from a different culture, a culture that is less refined than one's own culture, and so the migrant would make everything in one's homeland worse...

Interestingly these numbers are all for legal migration, and the article also says: "It should be noted that we have elevated incoming migration owing to our leading support for Hong Kongers and Ukrainian refugees, as well as very high numbers of students because of our flagship education sector.”.

The ONS breakdown is probably better compared to this dog-whistle article: https://www.ons.gov.uk/peoplepopulationandcommunity/populati...

> someone coming from a different culture, a culture that is less refined than one's own culture, and so the migrant would make everything in one's homeland worse

Why should one have to think a different culture and people are inferior, to want to preserve one's own culture and people? If it was environmentalists worried about native wolves being slowly displaced by an invasive breed of dingo, would you call them wolf-supremacists? Certainly nobody accused Kashmir of supremacy when they worried about immigration [1].

Or are human groups so unique, the first of their kind in the entire animal kingdom, that they actually benefit from a competing group moving into their territory?

[1] Kashmir’s new status could bring demographic change, drawing comparisons to the West Bank - https://www.washingtonpost.com/world/2019/08/08/kashmirs-new...