Usually you're on the only road heading to where you want to go. A diversion is going to take you many, many miles and out of your way and add hours to your trip.
There's a good chance you'll be delayed as well, and that can also take hours.
Narrow roads mean no spare space on the carriageway for the safety of those carrying out the work, so there are often convoy systems in operation to keep speed down through the roadworks.
We've had these signs just outside our village for a few months now and, for all the reasons listed above, I've sniggered every time I've gone through this diversion. If only all of them were as straightforward.