Here's why:

* Ribosomes are not organelles: Ribosomes are complex molecular machines, but they are not enclosed within a membrane like other organelles (like mitochondria, nucleus, etc.).

* Structure: Ribosomes are composed of ribosomal RNA (rRNA) and proteins. They are essentially two subunits that come together during protein synthesis.

* Function: Their primary function is to translate genetic information from mRNA into proteins. They do this by attaching to mRNA and using it as a template to string together amino acids into polypeptide chains.

In summary: Ribosomes lack a membrane and are essentially protein-RNA complexes involved in the essential process of protein synthesis.