I may be wrong, but I think there is also a cultural difference: everything here revolves around your gross income.

You'll hear "I win 100k a year", not "I win 181k a year". During hiring, you negotiate on the 100k number. Your contract mention this number. In the end, if there is a tax raise on the company part, they have to maintain your salary to the same level, even if it ends up costing them more than before.

That'd be also why US salaries seem so high from the outside, but are less impressive once your factor this in.