Often the line between hardware and software is blurred (is FPGA logic or processor microcode hardware?). Avoiding catastrophic failure may be a non-trivial problem. In the example case of a CNC machine, you must avoid putting too much strain on a tool, or suddenly colliding with the workpiece, thus requiring knowledge of the workpiece geometry. Software can also be easily updated to mitigate hardware faults or design problems (e.g. processor bugs).