Most people seem to conflate immigrants with refugees, which is false (refugees are a small minority of all immigrants), to see immigrants as poor, hard-working people who just need one chance, which is also false (most immigrants are skilled workers, generally not the poor of their country, and most immigrants you see are probably family members of other skilled workers), and to view immigration as a leftist cause (again not true, immigration is something that's mostly lobbied for by corporate interests).
But many more people feel opposed to migration for entirely different reasons: personal hardships directly due to immigration (loss of job or house), political leanings (pro-working class can mean anti-immigration), social cohesion/sustainability/community considerations, etc.
Not empathising with those people and having a reductionist view of immigration, which often stems from simply being shielded from any negative effects of it, as most on this forum are, lets people take a position like yours, which further polarises society and plays into the cards of the powerful you seem to oppose.
We probably agree on almost every political view apart from this.