Because, if you are a knowldege worker, even if you have the same job title you are not doing exactly the same job. There are really two choices here:

- allow people with the same job title (e.g. "Software developer") earn different salaries

- introduce fine grained competence levels (e.g "Software developer, level 23") and require promotion, based on detailed performance review, for every single request for a raise.

I prefer the former, because I don't like bureaucracy, but non-discrimination advocates are pushing towards the latter.