In games like AoE3, an economy is not just the amount of resources acquired over time, but also the quality and cost effectiveness of what you want to do with the resources: producing units. The French economy profits from two things at the same time: a good resource generation as well as a large variety of cost and pop efficient units. In the early game, the former is more impactful (e.g. via better vill shipments). The longer the game goes, the better the relative cost/pop efficiency of French units becomes (upgrade cards, especially the one for cheaper cuirs).
The French economy is perceived as very strong because it does not have any downsides. While the German economy can profit from high resource generation, due to an effective vill limit of 139 instead of 99, its units are both less cost and pop efficient as French units (worse skirm due to lack of HC dmg card, worse cavalry due to pop inefficiency, worse artillery due to the French age IV card). The Ottomans have cost efficient units (good artillery, very cost/pop efficient musketeer, good cavalry), but they lack potential regarding resource generation as well as variety in its unit compositions. While all (?) other civs have their strengths and weaknesses regarding resource generation, pop/cost efficiency and unit variety, France just got it all.
The better your units are, the more resources you can either invest in future resource generation or simply stockpile.
"To counter the pike-boom, you must become the pike-boom." - Karl V. (HRE)