The service intervals were determined by marketing initiatives rather than engineering considerations. They determined that servicing might be extended to approximately 15-18k without incurring significant claims on the 100k warranty.
In actuality, even contemporary and entirely synthetic oils deteriorate. The pivotal threshold for the majority of engine oils occurs at approximately 6,000 miles or more. By 12,000, they are all essentially exhausted. I cannot remember the source of the study links (perhaps Bob is the oil expert), but there has been an investigation of used engine oils indicating that a significant reduction was observed at the 6,000-mile mark.