Generally when I hear people discussing fiber the big costs is actually running the cables and terminating and splicing the ends, the actual cable itself is relatively inexpensive from my understanding.
But with regard to stealing cable, you really can’t do much with a fiber cable on the secondary market besides using it for fiber, and you also would probably need to re-terminate it that can be very costly. Compared to copper or other type of metal cables that can be sold very easily for scrap metal. That can be melted down and made into something else or just reused as cable.