Why do you measure Oil Tankers in dead weight whereas other ships in gross weight?

Oil tankers are measured in deadweight tonnage (DWT) because this metric reflects the total weight a ship can safely carry, including cargo, fuel, crew, and supplies, which is crucial for determining its cargo capacity. In contrast, other ships are often measured in gross tonnage (GT), which accounts for the overall internal volume of the vessel, providing insights into space utilization and regulatory compliance. The focus on deadweight for tankers emphasizes their operational efficiency and stability while carrying large quantities of liquid cargo.