Why Is Diffusion Insufficient to meet the Oxygen Requirements of Multicellular Organisms Like Humans?

In single-celled organisms, no specific organs for exchange of gasses are needed because the entire surface of the organism is in contact with the environment.
Multicellular organisms such as human beings have complex body designs and large body size. Because of that oxygen cannot diffuse into all the cells of the human body quickly.
Therefore, diffusion is insufficient to meet the oxygen requirement of multicellular organisms like humans.
Multicellular organisms (Human beings) bear specialized cells and tissues for performing various necessary functions of the body such as intake of food and oxygen, etc.