Because they're merely mathematical models of what we experience. They're no more "real" than man-bear pig, although they do have predictive capabilities.
Until they don't.
Ultimately what is real is a philosophical question. Why isn't man-bear pig real? The Imagination Land story arc really delved into this question.
No, the universe is actually made of tiny rubber balls, and I've seen rubber balls so I know they're real. Fields are just mysterious vibes, like ghosts that make your hair stand on end when they pass through you.
