Because passports have hard tracking and exit dates. To get a visa stamp, all countries want to know why you are there, where you will be, who you will be seeing and most importantly when you will be leaving. Illegals don't want any part of that and most probably don't even have passports. Remember, a very large part of the illegals coming in from Mexico are from Guatemala, El Salvador, Honduras and Nicaragua...they aren't even Mexicans.

Talk about a "border"....try getting in and out of China sometime....especially if you've been to the mainland and then are flying out of Hong Kong. They literally have posters reminding you that they will straight out kill you or put in prison for life for any and all monkey business. It's a fun time and hey! Free prostate exams!!!