why a vessel containing liquid always heated from below rather than from the sides

Please remember that hot fluid is less denser always rises to the top.

Water is a vessel is always heated from the bottom because it will result in proper and uniform heat transfer throughout the liquid due to the convective upward motion of water upon being heated from the bottom surface. If the water is heated from the sides this would not happen and would result in uneven heating and the process would be inefficient.