Round shapes distribute pressure more evenly and help maintain structural integrity, especially at high altitudes where there is a difference in air pressure inside and outside. 

The circular shape is inherently stronger than other shapes, particularly at the corners. This structural advantage helps prevent cracks or weaknesses in the window. 

Round shapes are easier to manufacture and install than more complex shapes. This simplicity in design and production contributes to cost-effectiveness in the manufacturing process

The circular shape minimizes stress concentrations, which is crucial for resisting fatigue throughout many pressurization cycles that an airplane experiences during its lifespan. 

Round windows allow for a uniform distribution of stress along their perimeter. This uniformity is important to ensure that stress from pressurization is evenly distributed.