Why is systems thinking called a "metaconcept" with roots in many disciplines rather than one theory?

Because it isn't a single doctrine but a cross-disciplinary way of seeing, assembled from biology, physics, sociology, psychology and more — and still evolving.

Systems thinking pulls ideas from wherever wholes-with-interactions show up:

• Biology — Maturana and Varela's autopoiesis: living systems continuously produce and maintain themselves (self-organisation).
• Physics / philosophy — radical constructivism (Heinz von Foerster, Ernst von Glasersfeld): what we "know" is constructed by the observer, not read off a ready-made world.
• Communication / therapy — the Palo Alto school (Bateson, Watzlawick, Satir): behaviour only makes sense in its relational context.
• Sociology — Luhmann's systems theory: society as self-referential communication systems.

Because no one field owns it, there is no single correct system definition or model. That openness is the point: it's a flexible "Denk-Alternative" (alternative way of thinking) to the usual linear, purely rational logic — a lens you apply, not a formula you memorise.