1. I'm not sure if people are universally incapable of talking like that, ie. with a bit of self-restraint and without cutting in at every opportunity. I'm not sure, but I strongly suspect that this is related to the culture of people involved. As an example (or an anecdote) I work remotely full-time and participate in a lot of conference calls, yet I was interrupted just a couple of times. Never twice by the same person, though.
2. People are able to talk (and communicate) in so many different circumstances and environments, like over the radio, on a boat during a storm, on paper, with a flashlight and Morse code... that I don't think it's that hard for them to assimilate yet another protocol to follow when talking. It really shouldn't be impossible, I think.
I really strongly suspect it's a matter of culture. I was taught, from a very young age, that interrupting someone when they're speaking is simply rude, and that if you respect someone enough to talk to them, you should also respect them enough to let them say whatever they wished to say.
It's also true for many of my acquaintances, at least those from my generation and older. People who talk too much, too often and are unable to let others finish what they're saying are regarded as hyperactive, perhaps with ADHD, or really bad marketers, sect recruiters and such. This is still just an anecdote, but it at least shows that it's possible for groups of people to use a different mode of talking.
An honest question: listening what the person has to say from start to finish is so tightly correlated (in my mind) with respect for that person, that I never considered that you can get interrupted, possibly many times in a row, and not feel offended. What do you think about this notion? Do you think that it's a speaker fault if he's out of words for a few seconds, and is responsible for maintaining a steady stream of speech from start to finish? In other words, how do you justify people cutting in? Or is that simply a non-issue to you?