Here's why:

* Endosperm is a nutritive tissue that develops from the fusion of a sperm nucleus with two polar nuclei in the central cell of the female gametophyte (embryo sac).

* This process, called triple fusion, is a characteristic feature of angiosperm reproduction.

* The resulting endosperm provides nourishment to the developing embryo within the seed.

Other plant groups like gymnosperms, ferns, and mosses do not have endosperm. They have different methods for providing nutrients to the embryo.