And some guards can be bribed. As Luiz Gonzalez has noted, you only need a very small hole for a lot of ants to get in the house. [His recommendation is to lock up the sugar.]
Have you actually checked how much the union rate for building 6,000 miles of security fencing would be? You may be seriously underestimating the cost.
Think about the Big Dig in Boston before you make your estimate.
Someone posted that the Israelis only had to build 72 miles of fencing.
There are only 6.7 million Isrealis.
you're right, of course. So lets say we only stop 80% of the invaders. Is that such a BAD thing? Union labor? Who need union labor? we have the SeaBees and the US Army Corps of Engineers. We have prison labor. We have hundreds of thousands of illegals sitting on their butts in jail. We have contactors who are employing illegals and can be court-ordered to work for nothing as part of their restitution for employing illegals, in lieu of jail or closing them down, using those same illegals prior to deportation. The possibilities are almost endless. Further, the conscripted labor of employers of illegals and their illegal employees, then deportation, would yank the incentive from those who would otherwise be repeat offenders. You just have to think a bit unconventionally to see the possibilities.