US incomes in general are extremely high at the median compared to all other developed nations save a few. The only comparables are nations like Australia, Norway, Sweden, Switzerland.

The median full-time job in the US pays around $45,000. That median person has no four year degree and is not living in a coastal city.

The demand is there, is I guess the most obvious reason. I don't think companies are paying good salaries out of the kindness of their hearts.