Here's why:

* Structure: Phospholipids have a unique structure with a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails. This structure allows them to form a bilayer in water.

* Bilayer Formation: The hydrophilic heads of phospholipids face the watery environment inside and outside the cell, while the hydrophobic tails face each other in the interior of the membrane.

* Membrane Flexibility: This bilayer structure provides the cell membrane with flexibility and fluidity, allowing it to adapt to changes in shape and size.

* Barrier Function: The hydrophobic nature of the tails creates a barrier that prevents the passage of most water-soluble molecules, while allowing the passage of certain molecules through specialized channels and transporters.

While other lipids are also present in the cell membrane, phospholipids are the primary structural component.