Earning potential is simply better in the US. It’s not just Mexicans that figured it out, but immigrants worldwide did. People with decent lives all over the world fly here and overstay their visas because the earning potential is just greater. The truth behind the ‘all immigrants are desperate’ narrative is most are not desperate, but ambitious, and literally making a business decision.

How many of those are from South America? My understanding was that most of that migration was from Central America, largely Guatemala and Mexico itself.

Among other things walking (or traveling in any way, really) through the border area between Colombia and Panama, including southern Panama, is tough-bordering-on-impassable, even relative to crossing the US border in the desert.

Mexico isn't in South America

Are you proposing that it is possible to travel by land from South America to USA without entering Mexico? Or perhaps the impoverished South Americans are buying yachts and tourist visas to bypass Mexico?

Mexico and Central America are not South America.

Mexico could be worse, but could also be better.