Because water towers are a cheap, reliable way of generating enough pressure to get the water into your house--not an issue with petroleum tanks. If you didn't have towers you'd have to use pumps, and buying enough pumps to meet peak demand would be prohibitively expensive for most towns. Towers simplify matters. You pump water up at a steady rate and gravity does all the work getting it down. Since the pressure is a function of the height of the column of water inside the tower, and since the height of that column doesn't diminish appreciably until the tank is virtually empty, the pressure stays steady regardless of fluctuations in supply and/or demand.