That FRA felt so quickly is a combination of many parts. Gamewise I would tie it mostly to "high command failures" -> bad decsions like spending all time money into Maginot/defense doctrine/tanks as Inf support role etc.."..
Without the Maginot Line France would probably have fallen to a surprise attack by Germany in 1939 (before Poland).
Had the Franco-German border lain a few miles further East, the river Rhine could have acted as a natural barrier across their frontier. Unfortunately, Germans actually live on both banks of the Rhine, so without it, so it's much easier for Germany to invade france than vice-versa. Assuming you get across the Rhine, a hundred miles of hilly, forested, sparesly populated terrain lies between you and any important German industrial/population centres.
Going the other way, like the Germans did in 1870, there's a few hills but like i say you're starting from a position west of the Rhine anyway and over most of the Vosges mountains. Most of the Iron ore and Coal that France has lies in this border region... and then you're into the nice flat plains of the Seine basin, not too far to Paris. Alternatively you can violate Belgium like in WW1 and just come in across the Franco-Belgian border. The rivers and Marshes are quite a barrier, but fortunately they'll only have the relatively small Belgian army defending them. The actual border with France is nice and clear and flat, and a large chunk of Industry sits right on the border, in the town of Lille.
Finally, you got to look at the structure of the French army. Because of their smaller population, they needed a largely reservist army to compete with Germany for size. But, reservists take time to call up. In a surprise attack, Germany could seize the hills of Alsace Lorraine before France has finished mobilizing. Then France would be trying to defend against a stronger attacker on flat plains, with much of the raw materials to feed their industry gone.
French divisions were structured with a majority of reservists but a core of full timers to act as the "glue". This meant that in peacetime, most of the army's full time contingent were in these "skeleton" divsions.. so very few divisions in peace time were actually full strength and combat ready.
This is not to say mistakes weren't made in the run up to war and that a human player couldn't do it better, with the benefit of hindsight. What i tend to do -
Hearts of Iron 2 / Arsenal of Democracy
1. extend maginot line slightly to belgian border. no more breakthroughs in the ardennes for you mr hitler
2. dont build any new navy
3. disband bomber force
4. build plenty of fighters, obtain radar blueprints from UK
5. don't build tanks, instead churn out huge amounts of motorized infantry
6. research best possible antitank weapons and attach to every division
In HOI 3
1. Extend Maginot line to Belgian Border/River Meuse
2. Build Radar across Maginot Line
3. And lots of fighters
4. Dont build any ships or Bombers
5. Get a large amount of motorised infantry for the dash to belgium
6. Research the best possible antitank weapons and attach to all divisions
7. Later, when your war economy means you have plenty IC and not much manpower, start attaching heavy armour to everything