You're assuming there is a fixed pie. What if the company is growing?

It's still zero sum for the current employees. Only the following scenarios are possible whether a company is growing or not:

  1. Nobody gets the promotion
  2. Somebody outside the company gets the promotion
  3. Somebody inside the company who isn't me gets the promotion
  4. I get the promotion

With the exception of #1, I only get the promotion at the expense of my coworkers (who want and are qualified for the job).

Edit: Put a better way, it is zero sum because you and your coworkers cannot equally share in the benefits of a promotion.