But if I played 1. d4 and you played 1...d5 then I played 2. e4 and you played 2...e6 ?
Then you've established the French Defense 'cuz you started it.
...so if you instead played 2...dxe4 then it would not be an attacking move because White started it, right? You just defend by taking a pawn.
When the opening is called "french defence", it means something else. Since white moves first in a game, it's widely considered that all "defence openings are started by black. For example, besides the french, you have the indian defences, and which one is played is totally up to black, not white. You have a common defence.
And few people call it the "french defence" anyways. Many people just call it the "french."