One reason: it's traditionally how our industry CRM's are set up. The ad source gets attached to the first in lead. The lead stays in "active" stage for months, even years. Customer happens to submit another online lead and the CRM matches to the customer. System triggers a "duplicate lead" and continues on with the original ad source unless there's a manual override. Reports by ad source are spit out of the CRM and dealer makes a judgement call from inaccurate reporting - however (in)accurate it was to begin with.
Dealers need to stop looking at their "advertising" sources as a one way channel into the dealership and begin to understand consumer behavior.