Spare ribs (also side ribs or spareribs) are a variety of ribs cut from the lower portion of a pig, specifically the belly and breastbone, behind the shoulder, and include 11 to 13 long bones. Meat and fat cover the bones.
Why are spare ribs so good?
Marbling. Baby back ribs contain less fat but are still tender due to coming from near the pig's loins. Spare ribs are a bit tougher with more cartilage that needs to break down, but they also have plenty of marbling because of their proximity to the pig's belly.
Any cut of meat that is expensive is expensive for exactly one reason: scarcity. On a pig, there are 2 racks of spareribs. No more, no less.