Why vent pipe not required for waste line?

A vent pipe is not required for a waste line if the system is designed to maintain proper drainage and air flow without it, typically in cases of short horizontal runs or when using specific types of fixtures that allow for air to enter the system naturally. Additionally, modern Plumbing systems often incorporate features like air admittance valves (AAVs) that serve the same purpose as a vent by allowing air to enter the drainage system while preventing sewer gases from escaping. However, local building codes should always be consulted, as regulations may vary.