Why can't america have a completely open border?

Many immigrants come to the United States seeking economic opportunities not present in their home countries. From my point-of-view, mass migration is the singular challenge of the 21st century. This is because it is a meta-issue that will affect our response to every other challenge. This is due to the fact that as mass migrations change demography, they may also affect changes in host nations’ cultures and political economies. The specifics of these changes are exceedingly difficult to forecast, because they hinge on dozens of variables specific to the migrants, the host nation, and the scale and rate of the movements. While we do not yet know the vector, the titanic, high velocity migration the West is currently experiencing will cause profound changes. Voters used to overwhelmingly favor less immigration, but opinions have changed fast amid an immigrant rights movement that took off in 2006 and partisan polarization driven by aggressive enforcement. In 1994, just 32 percent of Democrats and Democratic-leaning Americans and 30 percent of Republicans agreed that immigrants strengthened the country, according to the Pew Research Center. But by 2016, the share of Democrats who said so had skyrocketed to 78 percent. In 2006, 37 percent of Democrats said that immigration levels should be decreased, while only 20 percent said that it should be increased. In 2018, 40 percent said that it should be increased, with just 16 percent calling for restriction. The same trend has held on border politics: In 2010, 47 percent of Democrats said that they equally prioritized legalizing undocumented immigrants and “better border security and stronger enforcement of immigration laws,” while just 29 percent prioritized legalization alone. By 2018, the number prioritizing legalization alone skyrocketed to 51 percent.