They kinda did. The whole idea that France did not predict the attack through the Ardennes isn't exactly wrong, but it's oversimplified.
To understand the full situation, we must look at the geography of the area, as well as French experience in WWI. The Franco-German border, to simplify things, has easily accessible terrain on either end, on the Belgian border and in Lorraine. Here, the plains are open, the roads are flat, and armies throughout the ages have marched through them, going both directions. In the centre is mostly hilly terrain, with the giant Ardennes forest dominating the western half. The Ardennes was traditionally a deep, impenetrable wood that was difficult for armies to pass.
In WWI, the Germans managed to surprise the French by tearing up a five nation guarantee of Belgian neutrality and sovereignty. This opened up the least defended western flank of the French army, and resulted the debacle now known as the Great Retreat, which gave Germany control of most of Northern France and would have defeated France outright had it not been for the extraordinary effort of Allied armies at the Marne.
Less remembered were the first phases of the Battle of the Frontiers; while Germany was marching the long way around France's left flank, France launched two separate offensives, one into then German Lorraine, which was lured into German territory and then counterattacked vigorously, and the other through the centre at Ardennes.
The Battle of the Ardennes was a disaster for the French army. Commander Joffre miscalculated German strength, believing that German numbers in Lorraine and Belgium signified a weak German centre, when the German system of reserves and denuding the Eastern Front gave them more men to work with. 19 French divisions stumbled into 20 German, dug in and ready in the morning mist. The dense forest was difficult for cavalry reconnaissance and the French were thrown back with heavy casualties. Then Lieutenant Charles de Gaulle himself was one of the casualties in this battle.
Perhaps more than any nation, the Western Front was a traumatic experience for France; a whole generation of young men ground up trying to reclaim the most industrious part of the country that was occupied for four whole years. The interwar French strategy and planning was laser focused on preventing the debacle on the Frontiers from occuring again. The infamous Maginot Line was actually only half of the story. While the stationary defense of the Maginot Line held the ground, the mobile army, consisting of the best equipped, youngest and most motivated troops was supposed to bring mobile barricades and construction equipment into Belgium, to counter a German flanking attack. This would allow the French, who were outnumbered by the Germans, to fight effectively.
The Ardennes was the lynchpin that held the mobile and defensive sides together, like the hinge of a door. The French knew that the Germans had based their mobile army around armoured force, and they knew the Germans had a doctrinal preference for flanking attacks. Some French surveys of the Ardennes had warned of the possibilities of driving an armoured column through the forest, but that information did not make it into French military orthodox. When the French found out the Germans had marched into the Ardennes, they thought the woods and the garrison troops would buy them more time; that was all that was expected of them, but between an underappreciation of the power of the new tanks, an outdated system of communication that supported an overly rigid system of command and control (all topics of interest in themselves), the French failed to pull their mobile troops out of Belgium and redeploy in time, resulting in the breakthrough, encirclement and capitulation of the French army. Every piece of the French defense system worked, but poor coordination and slow reaction doomed France.