Seems a little disingenuous. That's like saying every infantry patrol must end in contact. They're not attacking fixed targets like airfields and bridges. They're patrolling more or less like ground troops, looking for targets of opportunity. And opportunity doesn't always materialize.
Troops on the ground (i.e. not trainers) come with costs. They have to be supplied and defended in an environment where the enemy controls 1/3 of the country, and the front lines are undefined. This was why the Iraqi occupation cost $50b a year, compared to the $3b the air campaign has cost so far. And US casualties would presumably rise to the hundreds per year that tanked the GOP's poll ratings.
From what I heard there are plenty of targets, but few that they are permitted to hit due to WH micromanaged ROIs.