Negotiation is entirely irrelevant. Why should a 2-5x variance in individual compensation factor into whether or not companies form a cartel to suppress all salaries?

Because there needs to be a trickle to start the flood. All companies are incentivised to co-ordinate prices to some degree, it's the desire to fill a particular role quickly or poach a particular superstar that causes companies to break from whatever happens to be the"industry standard", and those variances become factors in future negotiations. If there was never any variance in individual pay there'd be no pressure to adjust salaries at all, barring collective action. Which has its own negative externalities.