Here's why:

* Epithelial tissue is the lining tissue of the body. It's made up of tightly packed cells that form sheets, covering surfaces and lining cavities.

* Glands are specialized epithelial cells that produce and secrete substances. They can be either exocrine (secreting substances outside the body, like sweat glands) or endocrine (secreting hormones directly into the bloodstream, like the thyroid gland).

While other tissue types can contribute to gland structure (like connective tissue providing support), the functional, secreting cells themselves originate from epithelial tissue.