Here's why:

* Epithelial tissue is a type of tissue that forms the covering of all free surfaces of the body, both internal and external.

* It forms the lining of the digestive tract, respiratory tract, blood vessels, and many other organs.

* It also makes up the skin, which is the body's largest organ.

Epithelial tissue is characterized by its tightly packed cells with little intercellular space. It performs several important functions, including:

* Protection: It acts as a barrier against physical, chemical, and biological agents.

* Absorption: It absorbs nutrients in the digestive tract.

* Secretion: It secretes substances like hormones and mucus.

* Excretion: It eliminates waste products.

* Filtration: It filters substances in the kidneys.

* Diffusion: It facilitates the exchange of gases in the lungs.

While other tissues like connective tissue, muscle tissue, and nervous tissue also play crucial roles, epithelial tissue is the primary tissue responsible for covering and lining the body's surfaces.