Accuracy and truthful reporting would be extremely difficult to verify, and at least when it comes from the schools there's an aspect of accountability in how they report. We also think schools would find it hypocritical if an organization that aims for honest reporting is willing to accept information from unverified sources. It also duplicates the work schools already do in the way of data collection; of the nine components in the LST standard, career services offices already collect 7 of them for each graduate in answering the NALP/USNews surveys.
On top of that if I remember my 7th grade math properly you would be much more interested in the mean opposed to the median salary. Correct me if I am wrong, but MEDIAN would be like this if 7 salaries were reported as 25,000, 30,000, 35,000, 90,000, 100,000, 100,000 150,000, . The median salary would be 90,000. while the mean would be around 60,000. Reporting a median makes a school look a lot better if they can say 90,000 instead of averaging in people with very low salaries.