There are many people who come from other places as well that are also illegals and also stay and break the immigration laws not just south of the border.
Yeah it's a good point. The Mexican border has become the poster child for illegal immigration, but there are many others that come in with visitor visas , student visas, etc, and then simply end up staying and never go back.
To be fair though, the answer is to enforce it across the board, but this is not easy, because the US has a hands-off policy once you are here, you get to stay. We really only enforce things at the border or within 100 miles of the border. To my mind, local police should be enforcing it as well, if it's a law. This is what happens in other countries.