Because this notation is meant to show that you're dealing with ionic bonds.
For ionic bonds, electrons are assumed to be completely transferred; this implies that two atoms that form an ionic bond do not share electrons to form a bond per se, they are simply kept together by the electrostatic force of attraction.
The Lewis structure of sodium monophosphate looks like this
The sodium cations are not attached to the oxygen atoms that belong to the fluorophosphate anion because they do not share electrons with those atoms.
This means that electrostatic force of attraction that exists between the positive charges of the cations and the negative charge of the anion (notice that you have two +1 charges and one 2- charge) is the only thing that is keeping the cations and anion together.
A Lewis structure is used to show bonding between atoms in a molecule or formula unit. If two atoms share electrons, then that bond is represented by a line for every two bonding electrons.
If one atom donates its electrons and the other takes them, then the resulting cation and anion are represented without a line between them to show that electrons are not being shared.
So, covalent bonds between two atoms (both polar and nonpolar) are shown with lines, while ionic bonds are shown without lines, onyl with cations and anions drawn separately (usually close to each other).