many ways to die on the way to mount doom. Figured even his own orcs would not last just standing around mt doom. Certainly no human or halfling would even try to make the trek, especially if carrying the ring since the ring, once worn, is immediately seen by sauron, who could send several different horrible creatures immediately to come get you.

And yes, they did have armies strewn about the front and other areas of mordor except for the war with men going on.