We're just not smart enough. The really smart people (leftists) all know that bringing in ten million illegals that are often illiterate in their native language, much less English, benefit the economy. Millions of low and no skilled workers do not put downward pressure on wages, and millions of new households do not put upward pressure on housing costs. Millions of new people using the same public services such as health-care, education, food stamps, welfare and all other government assistance programs does not in any way increase the cost of those programs or reduce the availability.
Only really stupid people believe the impact is negative.