Because within the context of District 12 Katniss is a POC.
People from the Seam are:
Poorer than people from town
Segregated into their own neighborhood
Have lower-quality housing
Work harder, more dangerous jobs
Often have to resort to crime just to try to make ends meet
Earn no equity from their work - people in town work in shops that they pass on to their children, people in the Seam work for the mine and pass nothing but poverty on to their children
Are strongly discouraged from inter-marrying with people from town. When an intermarriage does occur, the town person is disowned from their town family and exiled to live in the Seam.
Are visually distinct from people in town because of their skin color
"White people can have olive skin!" Yeah, sure, that's a valid point, if "olive skin" are the only two words you ever read from the entire Hunger Games trilogy. If you actually read the entire book, however, it is abundantly clear which group within District 12 holds the position of relative privilege that white people do in our modern world, and which group holds the disadvantaged position that POC do in our modern world.
You can close your eyes and stick your fingers in your ears and chant "la la la I can't hear you Katniss is never explicitly said to be POC" as much as you want. You may as well add on "I don't see race, I just see people," and "All lives matter," and other slogans of the deliberately ignorant.
If your enjoyment of THG would be ruined by imagining Katniss as indigenous American, or as black, or as Melungeon, or as some kind of generic non-specific brown-skinned POC... If you can only enjoy the story if you can reassure yourself that it's about a white person... Well, do I even need to finish that sentence?
