Like others have said, proficiency levels are hard to define and highly subjective.
If we let people pick their proficiency levels themselves, there will be a lot of bias. Someone who rates him/herself "Expert" might actually be less proficient than (a more humble) person who rates him/herself "Intermediate". I think people in general are terrible at judging themselves.
Now, if we let others decide, we run into another problem: how do we determine the validity of their opinions? Or what if someone purposefully rates somebody else lower than they should, so they end up with a higher relative rating? This is not feasible either.
We kind of a system that handles this: reputation, which according to the FAQ:
Reputation is a rough measurement of how much the community trusts you; it is earned by convincing your peers that you know what you’re talking about.
While high reputation doesn't mean expertise, it's useful in gauging a user's contributions to the site and the helpfulness of their questions and answeres. Note that communication skills are important here: someone who is an expert but is unable to communicate their ideas clearly isn't going to be very helpful.
The system isn't perfect, but it's good enough. If you have better a idea, feel free to suggest it. Unfortunately an "expertise" field is not one of them, for the reasons mentioned above.