Here's why:

* Enucleation: During their development in the bone marrow, red blood cells eject their nucleus. This process is called enucleation. This allows them to be more efficient at carrying oxygen, as the nucleus takes up space.

* Specialized function: Red blood cells are highly specialized for oxygen transport. They lack other cellular components like mitochondria and other organelles, including the nucleus, to maximize their carrying capacity.

While they lack chromosomes, red blood cells still contain DNA. However, this DNA is not organized into chromosomes and is not used for cell division.

It's important to note that immature red blood cells (reticulocytes) do have a nucleus, but they lose it before becoming mature red blood cells.