Here's why:

* Simple diffusion is the passive movement of molecules from a region of high concentration to a region of low concentration. It doesn't require energy or the help of any membrane proteins.

* Small size: These solutes can easily slip through the phospholipid bilayer of the cell membrane.

* Lipid solubility: These solutes dissolve in the fatty acid tails of the phospholipids, allowing them to pass through the membrane.

Examples of solutes that move by simple diffusion:

* Oxygen (O2)

* Carbon dioxide (CO2)

* Steroid hormones

* Fat-soluble vitamins (A, D, E, K)

Important Note: While simple diffusion is a very important process, it is not the only way for molecules to cross the cell membrane. Other processes like facilitated diffusion and active transport are also crucial for different types of molecules.