[AnimalMuppet]

Optimistically: Empathy. Care for all humans, not just for the ones in this country. It is not wrong to look at life in America, life in the third world, and think "wouldn't it be nice if everybody could have this instead of that?" (But cynically, I suspect that most of them aren't thinking through the second-order consequences of that.)

Cynically: Greed and cynicism. Greed first. The tech workers who wanted to come here have always been able to come, so open borders are not going to increase the supply of tech workers much. But it is going to increase the supply of low-level workers, which results in lower prices in a number of areas. That makes tech workers better off, because at least some of the stuff they buy is cheaper.

The cynicism part is cynicism about America. They see it as nothing special, as having no special character worth preserving, as either neutral or negative overall. So if mass immigration happens, and it changes the character of either politics or culture, who cares?