Here's why:

* Covalent bonds involve the sharing of electrons between atoms. Polyatomic ions consist of multiple atoms held together by the sharing of electrons.

* Ionic bonds involve the transfer of electrons from one atom to another, forming ions with opposite charges that attract each other. While polyatomic ions have an overall charge, the individual atoms within the ion are held together by shared electrons, not a complete transfer.

Example:

The sulfate ion (SO₄²⁻) is a polyatomic ion. The sulfur atom shares electrons with the four oxygen atoms, forming covalent bonds. The overall charge of -2 is distributed across the entire ion.

In summary: Polyatomic ions are held together by the sharing of electrons, characteristic of covalent bonds.