Here's why:

* Phospholipids are the main structural component: They form a bilayer, meaning two layers of phospholipid molecules arranged with their hydrophilic (water-loving) heads facing the watery environments inside and outside the cell, and their hydrophobic (water-fearing) tails facing each other in the center of the membrane.

* Bilayer structure creates a barrier: The hydrophobic nature of the tails creates a barrier that prevents most water-soluble molecules from passing through freely.

* Flexibility and fluidity: Phospholipids can move laterally within the bilayer, giving the membrane flexibility and fluidity.

While other molecules like proteins and cholesterol are also important components of the cell membrane, phospholipids are the fundamental building blocks.