Because they are bunch of open borders advocates who have little connection with the reality that we can all see every day in America.
From the issues page of the Libertarian National Committee, Inc. – 2007
The benefits of open immigration
BY MICHAEL TANNER
America has always been a nation of immigrants. Thomas Jefferson emphasized this basic part of the American heritage, taking note of “the natural right which all men have of relinquishing the country in which birth or other accident may have thrown them, and seeking subsistence and happiness wheresoever they may be able, and hope to find them.”
The Libertarian Party has long recognized the importance of allowing free and open immigration, understanding that this leads to a growing and more prosperous America. We condemn the xenophobic immigrant bashing that would build a wall around the United States. At the same time, we recognize that the right to enter the United States does not include the right to economic entitlements such as welfare. The freedom to immigrate is a freedom of opportunity, not a guarantee of a handout.
A policy of open immigration will advance the economic well-being of all Americans. All major recent studies of immigrants indicate that they have a high labor force participation, are entrepreneurial, and tend to have specialized skills that allow them to enter under-served markets. Although it is a common misconception that immigrants “take jobs away from native-born Americans,” this does not appear to be true. In 1989, the U.S. Department of Labor reviewed nearly 100 studies on the relationship between immigration and unemployment and concluded that “neither U.S. workers nor most minority workers appear adversely affected by immigration.”
There is more here.