There are plenty of cables out there that are essentially just long pieces of metal protected by a rubber sheath. Their job is to carry a voltage, and they do. Thunderbolt (and presumably other, non-branded versions like Sony’s) is a little different. It seems that not all the horsepower is in the port; some has to be offloaded onto the cable itself.
This teardown at iFixit shows it’s not just the Apple Tax that makes the cables expensive, though what exactly is going on in those little chips is still mostly unknown.