Because you only need two dimensions to describe it; length and width.
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what are the 2d shapes of the real world
the cylinder has 2 2d shapes: the 2 circles on the end
Polygons are 2d shapes whereas polyhedrons are 3d shapes
2d shapes only have length and width, while 3d shapes have length, width and depth.
It will look like a 2D equilateral triangle if it is a triangular based pyramid which is called a tetrahedron.