Consider this, they get a large supply of some of the best heavy cav in the game with their lancers/noble/chivalric knights, they can also recruit heavy cav in cities with gendarmes, they have the best archers in the game with Scot's Guard, best crossbowmen in the game with Aventieurs, some of the best cav archers in the game with French Mounted Archers, and while their infantry isn't exceptional, it's strong and reliable, with dismounted feudal knights and getting armoured sergenats early in the game, which is something the English who are supposedly strong in infantry completely lack.
The French can recruit a strong and versatile lineup of troops from both cities and castles, and as well as having very strong traditional units, they aren't lacking in lategame gunpowered either.
Of course you would expect French to have strong cavalry, but they definitely seems to be the master of all trades. Thoughts?