Here's why:

* Egg yolk is a complex mixture of substances, including fats, proteins, and other molecules. It's not a single, self-contained unit like a cell.

* Eukaryotic cells have distinct, membrane-bound organelles (like a nucleus, mitochondria, etc.) that carry out specific functions. Egg yolk does not have these structures.

* Eukaryotic cells are the basic building blocks of multicellular organisms, while egg yolk is a component within an egg, which is itself a single cell.

The egg itself is a eukaryotic cell, but the yolk is just a part of its contents.