Cartels control the Mexican side of the border. How do you think anyone gets across? They either pay the cartels or work for them, as lookouts, prostitutes, drug mules, etc. The migrants are a revenue stream for the cartels and corrupt Mexican officials.

https://theconversation.com/migrants-at-us-mexico-border-must-get-past-cartels-before-their-long-journey-ends-129403 https://www.thedailybeast.com/how-mexicos-cartels-are-behind-the-border-kid-crisis https://www.winknews.com/2019/11/17/cartels-prey-upon-migrants-who-wait-to-cross-us-border/ https://www.npr.org/2011/12/04/143025654/migrants-say-theyre-unwilling-mules-for-cartels