I'm probably the only person reading this thread who owns a backhoe, and it's only a small one. There's really no material that will resist being dug through and is also practical to bury over miles and miles of fiber. There obviously are processes and measures in place to prevent "backhoe-fade" events, including armored cable, tracer wires and tape above the main cable but humans being humans things sometimes don't work out. Contractors can be liable for the cost to spice repairs which will be very costly so they're incented to not cut fibers.