Here's why:

* Structure: Phospholipids have a unique structure with a hydrophilic (water-loving) head and hydrophobic (water-fearing) tails. This allows them to form a bilayer in water, with the heads facing the aqueous environment and the tails forming a hydrophobic core.

* Membrane Formation: This bilayer is the fundamental structure of cell membranes, acting as a barrier that controls what enters and exits the cell.

* Fluidity: The phospholipid bilayer is not rigid, but fluid and flexible. This allows for movement of molecules within the membrane and for the cell to change shape.

* Function: Phospholipids also provide a framework for membrane proteins, which carry out a variety of essential functions, including transport, signaling, and cell-cell interactions.

While other lipids like cholesterol and glycolipids play important roles in membrane function, phospholipids are the primary structural component that defines the cell membrane.