European salaries are lower in part because Europe has less inequality than the US. It's a feature not a bug. That's the main point.
On an individual level I'm all for people negotiating, and for transparency in salaries so the workforce knows what is fair.
Actually you can; you can have a large group of people learning a lot more than the median income, arbitrarily more, counterbalanced by an equally large group of people earning barely less than the median income. This does in fact happen in practice and is precisely the reason the median income is so much lower than the per-capita GDP.
Inequality means that the distribution over available resources is uneven. Inequality can occur in the presence of growth if the gains from growth are not distributed equitably. I.e. inequality does not imply zero sum.
To the extent that economics agree on anything they seem to agree that wage growth has stagnated since, roughly, the 1970s, inequality is rising, and gains from growth are disproportionately going to the richest.
Finally, the census measurements capture data at an instant in time. At any one instant there is a fixed amount of $s in the economy and it's a mathematical fact that comes from the definition of median that you cannot have samples (i.e. income) higher than the median without having samples lower than the median.
Otherwise, who exactly are you shafting by asking for a higher salary?