Because the top performer most likely doesn't make the most money. They might also have a better negotiating position if other companies are pressured to do the same and reveal they pay better for a similar performance.
All of this is somewhat of a moot argument though as individuals hold very little bargaining power in the general work force. You might have some leverage when applying for a C-level position (but let's be honest, you don't go through a regular hiring process for positions like that) but if you want anything that's a significantly deviation from what the company will happily give you, you'll get nowhere without collective bargaining (and in fact you will more likely just get replaced). Individual workers don't hold much power, even if they're made to feel like celebrity athletes with foosball tables, fruit bowls and free drinks on tap.
