In the US I don't know why the gap is high, quite frankly, but in China it's pretty apparent why.
In the US I guess the gap is high due to people earning well, not robbing the poor blind like the robber barons of the 1800s (JP Morgan etc.) or of present-day China.
But, that said, it's not a good trend especially when the super-rich wall off their communities (and don't get me wrong, I lived like that recently) -- and that's a recent phenomenon. Prior to this, the rich would meet the poor somewhere, but if there is NO interaction except as server and served, then that leads to a lot of mistrust and antagonism
I think removal of the draft had an effect -- if the rich and poor served together as 18 year olds, when they turned 40, Mr. Moneybags would still remember that his chauffeur was his friend and the chauffeur would remember that -- lower social tension
AGain -- I'm not an economist, so these are just my thoughts and they could very well be wrong.
Rather than robbing the poor, JP Morgan saved the economy of the US on 2 different occasions.