That might be true, don't know. It could be that, as you say, they don't want to find them. Sufficient technology might not be available or they haven't allocated any funds for it.
As to technology already here what is there (1) GPR was available and is pretty good up to a depth of 40 feet or so and it has to be moved over the area of interest. (2) Seismic wave detection has been played with for a decade or two and has issues with other disturbances (traffic, wind, other things like natural seismic activity, construction) and would require a network of sensors. (4) Electrical resistivity can also be used but again probes would need to be networked in and can easily be screwed with. (5) Microgravity has been kicked around but tunnel differential in the gravitational field can be around 10 microgals in a background of the earth field that is around 100 milliongals. There are others but has DHS been funded to employ them.
Myself I think we ought to be checking out what Israel is using. They have detected tunnels to a depth of 170 feet or so, but I think their priorities are obviously a bit different.