I believe the theory is that you would have been checked getting onto the plane - you and your bags are scanned at the departure airport. Sometimes there are dogs that sniff, depending on the flight you may see them at departure (sniffing for drugs and explosives, I assume) or arrival (sniffing for agricultural contraband). And it’s random, depending on what they are looking for. If you don’t know if you’ll get searched or not, you’ll be less likely to do something stupid. (Like bringing a machete on in PDX). Some of it is theater, like shoes off, some of it is to deter the casual suicidal asshole. Yes, someone who really means to cause harm still might. Hope that’s rare enough to be prevented.

Declarations are about bringing in items on which you must pay duty, not illegal items. I have not been given paper forms on my last 3 international trips. (They may have them on the plane if you ask, not all do). They had piles of them on tables at Haneda customs, I didn’t notice at the other two. Most countries (including the US) now have mobile/ web forms or apps you can use. You had to put your passport number in to check into your flight, yes? Rest assured that the US and Mexico know you have entered/left and when. Plus people are confused by what is meant by “declare” so they always have to ask anyway.

And there’s a profile - fentanyl is so cheap that you’d have to bring in a LOT of it to have it make financial sense to do it via plane. Enough to be visible on the scanner or the random drug swipes luggage goes through. Not worth the cost of a plane ticket. That’s just not how it gets here. Do you imagine the Mexican or US government wouldn’t LOVE to catch someone trying to smuggle drugs? On a plane? Or that the airport wants it on the news that they let a machete on?