Why is it so hard not to notice a bunch of guys digging a hole next to a highway at night and planting very large and heavy explosives ! ?
What am I missing here?
There's a lot of miles of highway out there.
The amount of highway you'd have to monitor. You'd have to revisit each stretch more often than the time required to dig the hole and bury the IED. That would take more surveillance assets than we probably have.
However it should be possible, with the right sensors, to determine that the earth near the roadway has been recently disturbed. It would seem that they could cover the routes they intend to use with these sensors not long before the convoy arrives at any point on that route. I could be wrong about being able to detect the disturbed earth though. If it's mostly just sand anyway, that would make it much more difficult.
Add this to other great mysteries:
Why can't they fix loose shuttle foam?
Why can't they stop illegal immigration through Mexico by simply building a wall and guarding it?
Why are Palestinian terrorists exempt from the War on Terror?
There are 39,000 police officers in New York City and something like 150,000 coalition troops in Iraq, which is roughly the same size as California.
You do the math.