For the same reasons I don't want anyone to actually propose to get rid of murder, rape or jay-walking: it's an unobtainable goal, and at some point, the increases in the state's grip on power and money no longer justify the decrease in crime.
I personally find the rates of murder, theft and jaywalking in my corner of the world quite acceptle. "Less" of course would always be "better", all else being equal, but all else is not equal if you spend additional money on intrusive measures. There is no murder crisis here.
Now, I do not really have an opinion on whether illegal immigration across the souther border of the USA constitutes a "crisis" (I don't live there, and am not American), but sense that many Americans don't think so. And anyway, it will never be "solved" to everybody's satisfaction.
This is genuinly a matter of political opinion and political majorities. The problem to be solved is not "illegal immigration", but what levels with which hooks and loops and quirks ends up being low enough to be accepted by enough of the voting population. A presidential decleration of "emergency" is exactly the wrong way to approach this. It' s a totally arbitrary decision, and the stated "problem" cannot possibly be solved that way.