Here's why:

* Prokaryotes lack a nucleus and other membrane-bound organelles. The endomembrane system is a network of internal membranes that includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and vesicles. These structures are all absent in prokaryotic cells.

* Prokaryotic cells are simpler in structure. They have a single, circular chromosome located in a nucleoid region, but this region is not enclosed by a membrane. Prokaryotic cells rely on their cell membrane for many of the functions performed by internal organelles in eukaryotic cells.

Therefore, while prokaryotes have internal membranes, they do not have the complex, interconnected network of membranes that defines an endomembrane system.