Why economists like immigration
The study of economics leaves a person with two strong impulses.
The Libertarian Impulse: Mutually advantageous acts between consenting adults should, absent externalities, be permitted. The ability to engage in such trades is how people in free-market economies achieve prosperity. When the government impedes voluntary exchange, it prevents the invisible hand of the market from working its magic.
The Egalitarian Impulse: The market economy rewards people according to supply and demand, not inherent worth. Markets often fail to provide people the ability to adequately insure themselves against the vicissitudes of life and accidents of birth. We should, therefore, look for ways to help those who end up at the bottom of the economic ladder.
Most economists feel both of these impulses to some degree. The difference between right-leaning and left-leaning economists is how strongly they feel each of them. Right-leaning economists have a stronger libertarian impulse, whereas left-leaning economists have a stronger egalitarian impulse.
Although some debates in economics come down to which impulse a person feels more strongly, on immigration the two impulses are reinforcing. The libertarian impulse says, let the American employer hire the Mexican worker, for it is voluntary exchange. The egalitarian impulse takes note that the Mexican immigrant is the poorest person involved in the situation, and he benefits from more relaxed immigration restrictions.
Here is a conjecture: Whenever a policy appeals to both the libertarian impulse and the egalitarian impulse, economists will offer a relatively united view, as they do on the topic of immigration.