Why Do Liquid-Carrying Trucks(Tanker Trucks) Have Cylindrical Tankers?

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No doubt, we must have come across tanker trucks usually carrying cylindrical tanks on maybe highways or around gas stations in our neighbourhood. Many of us still don't seem to know they come in various sizes, but I can assure you that we've all noticed one special thing.  They carry cylindrical tanks and not rectangular or square tanks! Come to think of it, wouldn't a rectangular or square tank be easier to construct?  Or apart from that, won't it be an easy piece of cake during transport? Now, these are questions people have asked themselves without actually getting an answer, so let's get answers to them.   Tanker Trucks  A Tanker Truck is a motor vehicle designed to carry liquefied loads, dry bulk cargo or gases on roads. These vehicles are categorized as heavy, medium, or light duty types, based on their specific load-carrying capacities. 

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They may be isolated/non-isolated, vacuum-sealed, or pressurized/non-pressurized. Types Of Tanker Trucks Heavy Duty These tanker trucks are known for their large load-carrying capacity. Ranging from 5,500 gallons to about 9,000 gallons of liquidified gas. Also with a gross weight of about 26,000 Ibs. Surely, these tankers must come with high and expensive costs. Medium Duty These tanker trucks with their cylindrical tanks offer capacities ranging from 500 gallons to 4,000 gallons, at gross weights below 26,000 lbs. Typical tanker load configurations of this type include gas, water, and fuel. Light Duty They are usually operated as frame-carried vehicles. These systems typically offer capacities ranging from 100 gallons to 1,000 gallons, at gross weights below 12,000 lbs. Why Do Tanker Trucks Have Cylindrical Tanks?
  Absence Of Weak Spots 
Using square shaped or rectangular tanks to carry liquid oil or gas isn't such a great idea. Due to their corners, weak spots are created, for pressure to be precise. Upon experiencing a great deal of pressure, the tank itself would fail. But using a cylindrical tank on the other hand, it has lesser flat sides and corners, thereby enhancing it's life span or usability.

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Extraction When a cylindrical tank is used in carrying liquid, fuel, or gas, extraction in typically made easier. As using a spherically or cylindrically shaped tank would allow for the easy flowing of the liquid to the bottom of the tanker for extraction. That is why kitchen and bathroom sinks are not entirely rectangular, as this would disrupt the flow of the material being extracted.
  Maintenance As I said earlier on, rectangular or square tanks have corners, which would surely disrupt the process of effective maintenance. As dirts or liquid would get stuck to the corners, making it difficult to rub off or clean. But with a cylindrical tank, things wouldn't get cranky or difficult, as no problem will be encountered while cleaning its spherical surface. Reference
Sciencing.com
Wikipedia