Here's why:

* Prokaryotic: Eubacteria are classified as prokaryotes. This means they lack a nucleus and other membrane-bound organelles found in eukaryotic cells (like plants, animals, and fungi).

* Simple Structure: Their structure is relatively simple, consisting of a single cell with a cell wall, cell membrane, cytoplasm, and a single circular chromosome.

* Microscopic: Eubacteria are microscopic in size, typically only visible under a microscope.

However, it's important to note that while they are single-celled, they can form colonies and exist in complex communities.