Here's why:

* The nucleolus is a specialized region within the nucleus. It's not membrane-bound, but rather a dense area within the nucleus where ribosome assembly takes place.

* Ribosomes are made of ribosomal RNA (rRNA) and proteins. The nucleolus contains the genes for rRNA, and the rRNA is transcribed and assembled within the nucleolus.

* Proteins needed for ribosome assembly are imported into the nucleolus. They combine with the rRNA to form the ribosomal subunits.

So, while ribosomes are ultimately found in the cytoplasm (and on the endoplasmic reticulum), they are made in the nucleolus, the "factory" for ribosome production within the nucleus.