Why Aren't Leaves Linear Shaped?

The short answer is, rectangle or square shapes do not serve the purpose of a plant.

Leaves perform several functions for a plant. They respirate, trap sunlight, regulate temperature, minimize water loss from evaporation and so on. The leaf shape, size and thickness have evolved over time to balance these conflicting requirements. For example, if the leaf is wide, such as a water lily, it can absorb more sunlight but it will lose more water in the heat.

Leaves compete with other leaves and plants for their share of sunlight. Tall trees adapt by adjusting the leaf angle to the sun while shrubs growing under trees become more efficient in harvesting low levels of sunlight.

Irregular shaped leaves have several advantages for a plant. It makes it easier for the plant to adjust angles, optimize exposure to sun, compete with other leaves and minimize damage from winds.