And also there is some recognition that its going to get really hard to grow them as much proportionately. AMD could increase its earnings a lot pretty quickly, but its hard for NVIDIA to grow like that for the same reasons it is hard for Apple to grow. You are already getting so many of the $ available to spend in that area.
And yet, when other companies have grown their earnings like mad, their price has outgrown it, driving their P/E up further, because the market expects higher future earnings. nvidia is reaping the rewards of the things built up to now, but isn't necessarily expected to continue to hockey stick.
Their P/E is approximately the same as the rest of the S&P 500 technology sector's average.