1. Opening borders and letting people move to more productive, richer countries will create more economic problems than benefits. One consequence is a "Brain Drain," where talent from poorer countries pour into richer countries, and as a result, poorer countries no longer have the labor supply or talent to maintain their own economies and infrastructure.
2. Opening borders will endanger existing jobs in the country with an influx of immigrants. Bernie Sanders does not support open borders because an influx of immigrants would decrease opportunities for the American worker.
3. Bringing more migrants into wealthier countries doesn’t stop the wealthier countries from taking advantage of migrants who have more to lose and more at stake. These migrants tend to become part of lower-paid job economies. For example, In Qatar, foreign workers make up 95% of the labor force, but these workers cannot leave their jobs or the country without their employer’s permission. 
Even if workers from poorer countries freely travel to richer countries for better opportunities, it is likely that the richer countries will capitalize on these migrants' work and still mistreat them.