Why are American songs so successful when they have so many compositional faults?
Success for American songs often relies on characteristics OTHER than compositional correctness.
For example, musically, a pleasing tune or a tight harmony can provide delight in a listening audience. Even discordant sounds, when presented to produce an emotional response or to make a statement can be regarded as musically important. Innovation is another opportunity for a song to "strike a chord" with the public.
America, by nature of its large population of immigrants, has changed existing systems, be they, political, social or musical. We have proudly held this to be display of fairness and growth.
Ethnic, religious and cultural ancestry helped early American composers and performers adapt older musical traditions. African chord progressions and styles of music morphed into American Gospel, Jazz, Blues and Ragtime. The British Isles immigrants brought their folk and religious tunes that evolved into American themes in Orchestral Music (Gottschalk, Copeland), Country Western, Blue Grass, and protest music.
American musicians, schooled in the best of European tradition and traveling through out the world, adapted what they learned for marching, popular and religious music.
Often a song is popular for what it represents and the lyric message. Consequently, even if a song is lacking in structure or doesn't follow traditional expectations, it's lyrics may inspire patriotism, love, or fervor that is reflected in it's popularity.
Here is a fact that may trouble those who strive for musical purity:
Popular is not the same as right.
Popular is not the same as technically sound.
Popular is not as good, important or the best.
Popular means that it sells, that it attracts attention, that it has been produced and marketed, successfully as reflected in sales and air time. This is part of the American dream too.
Popular means success, even if it is poorly composed and performed.