They are not part of what you negotiate, and if the rates change your take home pay doesn't change, but it was exactly in anticipation of arguments like this I pointed out that my claim holds even for total tax wedge.
Maybe for you its not a part of what you negotiate, but it surely is for your employer. In my country it is a typical to work based on self-employment and you can see it quite clearly even in advertised salary ranges: the amount you can get when self employed is equal to the sum of salary offered for employees and payroll taxes.
What do you consider "typical"? In Europe, Greece is by far a massive outlier with about 30% self employed". No other European country comes close (I think Italy is second, about 10 percentage points lower, but not sure). In other words, in no European country is it anywhere close to being more typical to be self employed than being employed. The vast majority of people negotiate a salary, not a self-employed rate.
That it is part of the calculation for the employer is irrelevant - so is office costs, admin overheads, software licenses, equipment, yet we don't consider those costs deductions from an employees salary and bump the "real" salary up accordingly.
