Here's why:

* Small Size: Oxygen and carbon dioxide molecules are very small.

* Lipid Solubility: They are also lipid-soluble, meaning they can easily pass through the phospholipid bilayer of the membrane.

* Concentration Gradient: The movement of these gases is driven by the concentration gradient, moving from areas of higher concentration to areas of lower concentration.

No energy is required for simple diffusion, making it a passive process.