Here's why:

* DNA Structure: Genes are segments of DNA, which is a long, complex molecule that contains the genetic instructions for building and maintaining an organism.

* Nuclear Envelope: The nucleus is a membrane-bound organelle within the cell that protects and houses the DNA. The nuclear envelope is a double membrane that separates the nucleus from the cytoplasm.

* Chromosomes: DNA is organized into structures called chromosomes. These chromosomes are located within the nucleus.

Exceptions:

* Mitochondria: There are a few genes located outside the nucleus, specifically in the mitochondria, which are organelles responsible for energy production.

* Chloroplasts (in plants): Plants also have genes in their chloroplasts, which are organelles responsible for photosynthesis.

Let me know if you'd like to know more about any of these aspects!