Hi Rick,
There are probably a number of reasons why the tool is not connected to ground........
Some machines use insulated spindle mountings. Some high speed spindles use ceramic bearings. Some spindles have insulated casings. The list goes on....
My spindle shaft is grounded so I only have one wire and don't need to connect a ground lead to the chuck but a lot of people with wood routers probably use double insulated spindles where the shaft is not grounded.
Tweakie.