Why all the openings of blacks named as defence?

@Sachit365 A good question. I'm not fully sure but my guess is that the "Counterattack" suffix is synonymous with "Defense" in a similar way that "Opening" is synonymous with "Attack". For this reason it appears that the Marshall Attack is also known as the Marshall Counterattack as it is actually an opening for Black.

Yes ur right u feel the same counterattack means a type of defence in an attacking way

The Traxler Counterattack is named such because it is a response to White's Fried Liver Attack.

Similarly, the Falkbeer Countergambit is named such because Black gambits a pawn as a response to White gambitting a pawn in the King's Gambit.