Why are there so many unauthorized migrants in the United States?

In this paper Emily Ryo says 

My findings show that perceptions of certainty of apprehension and severity of punishment are not significant determinants of the intent to migrate illegally; however, perceptions of availability of Mexican jobs and the dangers of border crossing are significant determinants of these intentions. In addition, individuals’ general legal attitudes, morality about violating U.S. immigration law, views about the legitimacy of U.S. authority, and norms about border crossing are significant determinants of the intent to migrate illegally. Perceptions of procedural justice are significantly related to beliefs in the legitimacy of U.S. authority, suggesting that, all else being equal, procedural fairness may produce greater deference to U.S. immigration law. Together, the results show that the decision to migrate illegally cannot be fully understood without considering an individual’s underlying values and norms.