I think part of the issue here is that (at least speaking for people like me from the US), we are very comfortable and accustomed to making advance reservations, standing in line, etc.
In South America IN GENERAL, people generally show up when they want and have more of a self-focused attitude about having their needs accommodated. There's not a lot of respect for waiting your turn, not crowding the person in front of you, or noticing when there are a dozen people waiting for a table and you've been sitting there with your empty cup of coffee and cell phone for the past half hour or more. That all said, I do love that you don't have to make reservations here a million years in advance to go to a good restaurant. And, it's pretty great not to have a check shoved in your face the minute you put down your fork.
Anyway, my husband is from South America, and he (as well as all of his family) is still is the worst about making an advance airline seat reservation when he is traveling without me. Luckily, he doesn't really care or complain if he gets stuck in a middle seat that doesn't recline by the bathroom and with a broken TV monitor.
I've noticed more than once on airplanes to and from SA that families do show up and expect that that everyone will move seats so they can sit together. I used to be a nicer person, I guess, but now I just ignore the requests over the sound system, because I did call way in advance (and wait semi-patiently on hold for forever) or go online to reserve my window seat so that there is even a slight chance that I will get some sleep. Luckily, no one has ever asked me directly to move, but I think I would say no. Get your act together people if you really want to sit together, make some advance effort. It may not be the Argentine way, but it's the way it works on airlines.