Here's why:

* Structure: Ribosomes are made up of RNA and proteins. They are very small organelles, and they don't have a membrane enclosing them.

* Function: Ribosomes are responsible for protein synthesis. They are found in both the cytoplasm and on the endoplasmic reticulum (ER). They need to be able to access the cytoplasm and mRNA to carry out their function, so a membrane would be an obstacle.

In contrast, other organelles like the nucleus, mitochondria, and Golgi apparatus are surrounded by membranes.