Why was the ground for this fluorescent fixture not connected to the ballast?
My guess would be that because the fixture has a metal frame, the ballast is grounded via its contact with frame.  If the frame were plastic, you would need to use the terminal on the ballast.

If you're converting it to hardwired, you should definitely connect the ground wire to the grounding screw.

You're also going to need to cut a hole if there's no punch-out available and make sure to use a cable clamp/strain relief suitable for using with romex.