Why keep out scientists, engineers and technological experts?

I quote from an article posted on the Internet by the Brookings Institution about myths related to immigration to the United States:

The United States makes a special effort to attract scientists, engineers and technological experts. Right now, we set aside only 65,000 of America's nearly 1 million visas each year for high-skilled workers. This is well below the 195,000 high-skilled visas that the U.S. allowed from 1999 to 2004. One study found that 25% of all the technology and engineering businesses launched in the USA from 1995 to 2005 had a foreign-born founder. In Silicon Valley, that number was 52.4%.

Why in the world would we want to restrict the number of highly-skilled workers coming to the United States? They clearly strengthen our economy, helping to assure social security for our dependents and creating the businesses that create jobs and wealth.