it's a complex issue.
Congress is convinced immigration will solve the SS/Medicare problem. They cannot, only the delay the inevitable changes or collapse.
Congress is convinced that immigrants will do jobs Americans will not do. That is correct as it pays more to stay home and do nothing and live off the system. With the surplus of low wage earners created by the government, we have stagnant wages. A problem government created.
Congress is convinced that the population decline in America can be corrected by immigration. The reality of people having 1 or 2 children is the result of government interference in the family, taxation etc. The I want more generation. Government and people create this problem.
This is just a few of the issues attached to the immigration problem that everyone tends to ignore.