Dry wall is smooth, but the most common size it comes in is 4 ft x 8 ft sheets. The seams that occur between the sheets are the problem. The seams are taped and drywall compound (sometimes called mud) is used to cover the seam. This is a very difficult procedure. To get a really smooth surface the mud must be sanded and repeated coats applied and sanded to feather out the seam. Otherwise you end up with a bump at every seam. Texture makes the seams easier to hide. To get a perfectly flat wall, a lot of man hours and skill need to be used.
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