But can someone explain to us GA folks why the FMS isn't always in use as an extra precaution?
The FMS is basically always set up and active in modern airliners, because it does exactly that: Manage the flight. This includes a lot more than the navigational part.
Concerning this particular flight, there would have been multiple possibilities to put the intended flight path, or an aide to it, into the FMS. What happens quite often during programmed approaches with a visual final part is, that the pilots, when they transition to this visual phase, no longer follow the programmed FMS magenta line and fly ..... visually. For one reason, the visual part might not be displayed/programmed entirely, or pilots divert intentionally due to approach paths not aligned with the runway.
What i am trying to point out: Even if the flight path is partially or entirely programmed in the FMS and displayed on the PFD, many pilots switch to complete visual paths for many reasons.
Therein lie the traps. In not doing so regularly (children of the magenta), they might then screw up ......
The easiest way to get some valuable help would be to program the landing runway into the FMS as the last waypoint, with an extension on the aligned bearing. Most modern FMS allow that with a 5nm final displayed as a magenta line on the PFD. This allows a nice line up for the visual final part, some 2nm before impact.
At this stage we do not know what was programmed and displayed in the AC 320.