Shown above are the results of making bends in sheet material. The diagrams could apply equally well to metals or thermoplastics, although the latter would need to be heated in the area surrounding the bend line.
You can see the problems that are caused in bending the material along a curve ~ either buckling or stretching ~ this is something you can see for yourself by trying to bend a piece of paper in the same way and is a problem that occurs when bending tubing.
Bends of a large radius are made using bending rollers: This process is reserved for sheet metal although thicker plate section can be bent if it is heated first.