[piytar]

Um, Mexico has an armed southern border: https://cis.org/Luna/Mexico-Sends-Troops-Its-Southern-Border

[InABunkerUnderSF]

I've crossed the southern border border between Guatemala and Tobasco, a river crossing, and between Guatemala and Chiapas on the Pan American Highway. In Tabasco the boat lands you on a military post and the army processes your immigration forms and searches your bags. In Chiapas La Migracion and Customs processes your documents and search your vehicle and you can drive on. But there is a battalion of infantry headquartered just south of San Cristobal de las Casas about 10 km up the road.

Unlike some countries, Mexico takes their national sovereignty seriously.

[21twelve]

According to statistics from a U.S. Citizenship and
From your link (2016 article):

Immigration Services (USCIS) the number of border asylum claims has multiplied 10 fold since 2009. The rise in the number of “credible fear” claims is not random, but follows a 2009 Obama executive directive implementing a “catch-and-release” policy. As a result of this memo, aliens are now paroled into the U.S. instead of being kept in custody while their asylum claims are fully reviewed, as required by law. In other words, U.S. immigration policies in the United States have created a pull factor, which criminal organizations exploit in order to feed their smuggling business.