Here's why:

* Eukaryotic cells are characterized by the presence of a membrane-bound nucleus that houses the cell's genetic material (DNA).

* Prokaryotic cells (like bacteria and archaea) lack a nucleus and their DNA is located in a region called the nucleoid.

So, all organisms whose cells have nuclei are classified as eukaryotes. This includes a vast range of life forms, from single-celled organisms like amoebas to complex multicellular organisms like plants, animals, and fungi.