Here's why:

* Central Atom: Phosphorus (P) is the central atom.

* Bonds: Phosphorus forms four single bonds with four oxygen atoms.

* Electron Pairs: Phosphorus has five valence electrons. It shares one electron with each oxygen atom, forming four single bonds. It also has one lone pair of electrons.

* VSEPR Theory: According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, the electron pairs around the central atom (both bonding and lone pairs) arrange themselves in a way that minimizes repulsion. Four bonding pairs and one lone pair on the central atom lead to a tetrahedral arrangement.

The lone pair on the phosphorus atom does slightly distort the shape, making the bond angles slightly less than 109.5 degrees. However, the overall shape is still considered to be tetrahedral.