1. Drugs. Our nation has a serious drug problem, with voice-damaging alcohol, tobacco, and one I don't know if I can mention being the lead culprits. Two of the drugs not only are damaging to the voice from dehydration, but also from building up gunk on/in all of the singing organs, and by causing cancer to the organs in question. The other drug, alcohol, causes dehydration, emotional numbness (thus poor emotional performance for the song), slurring, etc.
Yes, most American singers use 1+ of those drugs, including around their performances. Yes, it is a huge culprit in why they suck at singing.
If you want to be a good singer, avoid recreational drugs. If you want a good life, avoid people who use those drugs, too.
2. a) Americans have differing degrees of Social Anxiety Disorder, which causes degrees of stage freight. Being nervous tightens, and stiffens the vocal chords, and deepens the voice.
It raises question... Why are Americans trying so hard to fake being gregarious? Yet, we all have Social Anxiety Disorder to some degree from this gregarious-designed society.
2. b) Americans are especially nervous about what we say, because we're so... Easy to offend, outrageous when offended, and pair those up with our freedom to speech. This whole freedom of speech thing doesn't seem to be working out.