Here's why:

* Prokaryotic cells are simpler than eukaryotic cells (the kind found in plants, animals, fungi, and protists).

* They lack a nucleus and other membrane-bound organelles that eukaryotic cells have.

* Their DNA is located in a region called the nucleoid, which is not enclosed by a membrane.

So, while bacteria are living organisms, their cellular structure is fundamentally different from that of more complex organisms.