If that is true, then Canada should have exactly the same problem.
Mexico has been destabilized by U.S. influences fore decades and the widespread corruption is in no small due to the number of areas where honest cops and prosecutors are dead – there’s even a phrase “plomo o plumo” (Silver or lead?) describing the decision so many have been told to make. That’s a huge inflection point which is hard for any country to recover from.
There is a border crossing that literally just consists of some cameras and phones: https://www.atlasobscura.com/places/northwest-angle (admittedly to smuggle things through this you would have to take a boat to get there, and if you did this regularly someone would probably notice)
There are towns (and houses, and at least one library, also a restaurant) split down the middle with no barrier: https://www.atlasobscura.com/articles/the-incredible-complic...
And there are ridiculously many miles of basically uninhabited border.
It's probably "stronger" at the well traveled crossings, but only because you're not likely to be able to bribe our border guards.
For example, this guy crossed accidentally, and was promptly met by ICE agents.
> And you're really going to tell me that clearing the forests for 20ft at the border magically prevents people from crossing if they want to?
Yes... it's littered with ground sensors and cameras.
> Do you think there are thousands of drones patrolling the clearings or something?
No. Of course not. But there is a sophisticated system on both sides to deal promptly with these things.