I most certainly disagree with you in a few points.
First, you seem to assume that America (the continent) is just divided in three homogeneous blocks (by language). However, that is not true. While spanish from the river plate is technically the same language (and mutually intellegible) than other dialects (e.g. Mexican Spanish), there are lots of localisms that set them appart, usually making a word or two incomprehensible (needing further explanation). The same goes with words for animals and plants, as different words are normally used from the acceptable list, hampering conprehension. While there is a common subset, care must be taken not to use localisms that might lead to misunderstanding, sometimes with hillarious effects. See the case of cajeta for an example.
Second, you seem to be assuming (sorry if I'm wrong) that since America is divided into three big blocks, we don't bother to learn foreign languages, which is false. Here, in Buenos Aires, people above a certain educational level usually learn english as a second language (as it is very useful). There is also a fairly large number that learns portuguese (due to the proximity with Brazil). There's a similar effect in Brazil, swapping portuguese and spanish. While teaching at the local university, I expect the students to know english. The textbook is in fact in english (Peterson and Davies' "Computer Networks, a systems approach") and it is rare to find a student who can't understand it (less than one for each semester). Also, in my line of work (highly technical computer job), everyone in the Argentina office has at least some basic knowledge of english, while in the US office most cannot speak spanish at all.
Third, the urge to learn languages is hardly exclusive to Europe. Following your logic, I would have never learned french or japanese, as in theory I would have no use for it, something completely false, for the same reasons you expressed (people open up when you speak their language). In my case, tourists come here to visit, instead of me crossing some border close to my homeland.
There's something else that makes the people in the States not want to learn other languages. With a big border with Mexico, and a thriving latino culture in Miami and Los Angeles, I would have expected a high proficiency in spanish, were the conditions the same as in the rest of America. The same would apply to states in the vicinity of Quebec and french.
Interesting fact: I can understand Galician because it sounds like portuguese with spanish pronunciation (we get Galician TV on cable). I wouldn't risk trying to talk it, as I would probably take the wrong choices for words and pronunciation, mangling it beyond recognition.