Here's why:
* Publicity: Katniss understands the power of spectacle in the Games. Matching outfits create a visual impact, generating more interest and potentially garnering them sponsors. This could translate into vital supplies and resources during the Games.
* The Star-Crossed Lovers Trope: The matching outfits play into the "star-crossed lovers" narrative that Haymitch (their mentor) encourages them to embrace. By presenting themselves as a romantic couple, they can appeal to the audience's emotions and gain their support. This will likely translate into more sponsorships and potentially even public intervention to save them.
* Unity: The outfits symbolize their unity and commitment to each other, which could help them survive the Games. In the brutal arena, having a strong bond with a fellow tribute could be the difference between life and death.
While there is a slight risk that the Gamemakers might see the matching outfits as a challenge or a mockery of the Games, Katniss believes that the potential benefits outweigh the risks. She is willing to gamble on this strategy to improve their chances of survival.