Because an ARCHICAD Wall is either:
1. a single straight segment
2. a single curved segment
3. a single trapezoid segment
4. a single polygonal wall
The ARCHICAD Polyline element is an element that can consist of multiple segments. A Wall element is not like that. Try it: when you create a chain of Walls, ARCHICAD creates multiple Wall elements (usually grouped), not a single Wall element.
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