Why do PEOPLE who come into the UNITED STATES of AMERICA need to be properly VETTED?

Why do PEOPLE who come into the UNITED STATES of AMERICA need to be properly VETTED? https://www.msn.com/en-us/news/us/us-attorney-narrowly-escapes-knife-attack-by-illegal-immigrant-blames-new-york-s-sanctuary-policies/ar-AA1H7uGT?ocid=socialshare NATIONAL ASSOCIATION FOR THE ADVANCEMENT OF WHITE PEOPLE