Because of this:
"Give me your tired, your poor, Your huddled masses yearning to breathe free, The wretched refuse of your teeming shore. Send these, the homeless, tempest-tossed to me, I lift my lamp beside the golden door!"
It's the inscription on the Statue of Liberty. The US decided, long ago, that we'd be a haven for people of other countries who are being persecuted or who otherwise want a better life.
If we don't want to do that anymore, I suggest we tear down the Statue and replace it with a closed gate.
Besides that, we've been meddling in the affairs of Central and South America for decades with mostly-disastrous effect on their economies and political systems. Accepting refugees would be a small step toward acknowledging and working to right that wrong.