1. Lack of Membrane: Organelles are defined as membrane-bound structures within a cell. Ribosomes lack a surrounding membrane, instead existing as free-floating complexes within the cytoplasm or attached to the endoplasmic reticulum.

2. Simple Structure: Ribosomes are relatively simple structures composed primarily of ribosomal RNA (rRNA) and proteins. They lack the intricate internal organization and complexity seen in many organelles.

3. Not Self-Replicating: Organelles generally have their own DNA and are capable of replicating themselves independently. Ribosomes, however, are synthesized in the nucleolus and assembled in the cytoplasm, relying on the cell's own machinery for their creation.

4. Function as a Complex: While performing a vital function in protein synthesis, ribosomes are not considered individual functional units. They act as a complex of rRNA and proteins working together to translate genetic information into proteins.

Therefore, despite their importance in cellular processes, ribosomes are not classified as organelles due to their lack of a membrane, simpler structure, reliance on the cell for replication, and function as a complex rather than an independent entity.