Here's why:

* Prokaryotes are single-celled organisms that lack a nucleus and other membrane-bound organelles. Their DNA is located in a region called the nucleoid, which is not enclosed by a membrane.

* Eukaryotes, on the other hand, are cells that have a true nucleus and other membrane-bound organelles such as mitochondria, Golgi apparatus, endoplasmic reticulum, etc.

Therefore, the defining characteristic of prokaryotes is the absence of membrane-bound organelles.