Why are oxidation state and formal charge considered as separate entities?

Formal charge: The theoretical charge on an atom in a molecule, calculated by V-1/2B-L, where V is the number of valence electrons, B is the number of bonding electrons, and L is the number of lone pairs

Oxidation number:A fictitious system of measuring electron density and distribution. It is the hypothetical charge that would be present on each atom if a molecule was shattered into its individual constituent atoms with bonding electrons ending up with the atom of the bond having the higher electronegativity.
Thus they are different