Nobody negotiates net, but companies will keep in mind an employee's
lowest possible tax bracket given their gross (and therefore their
highest possible net) when determining the cost:benefit of offering marginal "material compensation" increases (vs other components of total compensation) — both during initial negotiation, and when trying to design later incentives.
Which is to say, if you're making USD$100k/yr working for Ericsson in Sweden (where you're in the highest, 52.9%, tax bracket for any marginal income above USD$83k), they're gonna be thinking differently about how to reward you as a high performer, vs if you were making USD$100k/yr at Google in the US (where that salary would instead just have you scraping the top of the 22% tax bracket.)
Google would almost certainly just give you more money — and would likely continue to do that indefinitely, as even the highest US federal tax bracket (37% at USD$600k/yr) isn't too onerous. Ericsson, on the other hand, might already rather offer you something non-monetary and "cheap" for them (e.g. a company car), rather than biting the bullet on a "substantial, productivity-incentivizing" raise (i.e. one that meaningfully increases your net income.)
And that is why precisely why salaries tend to be lower in many European countries (that have these high tax brackets); and why everyone in those countries instead expects / demands / enshrines requirements in law for tons of non-monetary benefits, like long vacations! "Purely salary" and "salary levels off, then tons of non-material benefits" are both Nash equilibria in compensation space; just very different ones.