Here's why:

* Prokaryotic cells are simple cells lacking a nucleus and other membrane-bound organelles. Examples include bacteria and archaea.

* Eukaryotic cells are complex cells with a nucleus and other membrane-bound organelles. Animals, plants, fungi, and protists are all made of eukaryotic cells.

Fleas are insects, and insects are multicellular organisms made of eukaryotic cells. They have specialized tissues and organs, which wouldn't be possible with prokaryotic cells.