Because that's the easiest way to make them, and the strongest. EDIT: Also the circular shape allows for a smoother flow of liquid within the pipe and less resistance.
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A round object with flat, circular ends is a cylinder. It is shaped like a can of vegetables.
They need not be. Many electromagnets are circular disks or toroidal (donut shaped). Bar magnets, not surprisingly, are bar shaped - often with a rectangular cross section.
There are various reasons that manholes are shaped like a circle. For example, it is much easier to dig a circular hole.
A conical shape is defined to be shaped like a cone. A cone is a hollow or solid object that dwindles from a circular or as close to circular as possible down to a point.
Decagon which has 10 sides Dodecagon which has 12 sides Drum which looks like a cylinder Dustbin which is a circular shape Door is usually rectangular shaped Drain cover is usually quadratical or circular shaped Disc is a flat circular shape Drinking glass has a circular rim Dime is a perfect circle Dice is a cube shape Dodecahedron is a 12 faced polyhedron Dollar is oblonged shaped Daily newspaper is normally a 4 sided quadrilateral Dimensions can be 2 dimensional or 3 dimensional