They shouldn't, but the way the system works, special interests donate to political PACs, and elected politicians feel obligated to pander to those PACs. Campaign Finance Reform means that the money stream simply gets rearranged. There is no political will to do anything about the illegal alien problem. It will take a national catastrophe like the introduction of an epidemic disease that has about a 50% mortality rate, traceable to illegals, or a WMD set off in the heart of Los Angeles, again traceable to an illegal southern border crossing, for the politicians to actually do something about this problem.
The agencies charged with dealing with the border are more interested in prosecuting those citizen groups which are patrolling the border in order to protect their properties than they are in stemming the tide. I think another approach would be to completely defund the INS and just open the border entirely. That way we could at least save several billions of tax dollars. (yes, I'm being facetious - sort of.)