one of the reasons is that kids are much more receptive to learning languages at a young age. Brain synapses that aren't developed die off. Children who learn it when they are small are at a distinct advantage over an adult learning it at the same time.
Studies have found that children in bi-lingual homes are a bit delayed in language at first. it's beleived that they are "sorting," but often soon exceed their uni-lingual peers. Lots of theories for that too. But it's not because they don't understand. Children understand far more than they can verbalize. THey are absorbing.
Languages change and evolve. If you are in a country b/c English is the language, then you should probably be in England. English became the language of the US because people came in using it. Native Americans didn't start out speaking English. As a matter of fact, Spanish was the primary language in many of our states before we took them over due to the Mexican-American war (
Texas, and
Colorado,
Arizona,
New Mexico, and
Wyoming,
California,
Nevada, and
Utah.) (wikipedia)
English is NOT a second language in most countries. Not officially, at least. It is taught because it has become the most international language of business. But you won't find many homes in other countries where English is what they speak at home. Most of those countries also teach more than English. My Italian ex, he spoke Italian, English, French, Spanish and some Japanese. America is probably the most ethnocentric modern country in teh world.
I would argue, however. that SPANISH is a second language in the US. It's the second most common, and is the primary language used at home by 34 million Americans 5 and older.