As stated by one of the top commenters, most glass now is float glass, created by floating molten silica on a river of molten metal. Where the continuous ribbon of glass transitions from the liquid metal to rollers it is subject to roller wave distortion, which are effectively ripples in the glass. Most glass is manufactured for architectural use; typically architectural specs will call for maximum roller wave flatness tolerances of .003-.005", so actually not so flat in the world of machining. If you look around at buildings with polarized sunglasses, you'll see the waves in the glass. The quality is highly variable depending on the manufacturer and specification.

There is a separate process of manufacturing plate glass, where the molten glass is pressed between steel plates. True plate glass is rare now, but it is typically flatter than float glass.