Last time we drove up and through Andorra, from France to Spain, the trip took a heck of a lot longer than we thought, so it may just be as mentioned above, that the route is too slow to be included in the route planner. We also got held up for a long time with the border check. We were staying in France (near Carcassonne) and driving a friend's French registered car that we'd borrowed, and this seemed to cause a bit of suspicion, as they made us completely empty the car to search everything. We'd (wrongly) assumed that the border with Andorra would be just like most other EU borders, but as Andorra isn't in the EU they still had conventional border controls when we drove through there.