Here's why:

* Phospholipids are amphipathic: They have both a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. This allows them to form a bilayer in aqueous environments, with the hydrophilic heads facing the watery environment inside and outside the cell, and the hydrophobic tails facing each other in the interior of the membrane.

* Phospholipid bilayer structure: This bilayer forms the fundamental structure of all biological membranes. It acts as a barrier between the cell's interior and the external environment.

* Other membrane components: While phospholipids are the primary structural component, membranes also contain other macromolecules like proteins and cholesterol. Proteins are embedded within the phospholipid bilayer, performing various functions like transport, signaling, and enzymatic activity. Cholesterol helps maintain membrane fluidity and stability.

So, while other macromolecules contribute to membrane function, phospholipids are the backbone, creating the fundamental barrier structure that defines cell membranes.