why do giant covalent structures not have intermolecular forces

Intermolecular forces are between different molecules.

Giant covalent structures are generally defined as one structure or molecule so it would be wrong to consider the intermolecular forces as the force keeping the structure together as a solid or a liquid.

However, there are intermolecular forces but they are negligible. There are many types of intermolecular forces and anything with a charge can have intermolecular force. Giant covalent structures included as they have electrons.
This is because electrons may be located anywhere in their orbitals at a given point in time. If they are located in one particular region just due to chance and time this may lead to the formation of a dipole as more -ve charge is in one positon. Which leads to electrostatic forces of attraction (think of magnets north and south interacting).

Why do graphite and graphene have intermolecular forces they are made of carbon, the same molecule?

They all have intermolecular forces? Every molecule that exists has intermolecular forces since they have electrons. Its just that we focus on the strongest forces and tend to ignore intermolecular forces where there is another one that is present.

Graphite and graphene are allotropes of carbon meaning the carbons are arranged in different shapes and joined by covalent bonds.
They are still giant covalent structures and they still have intermolecular forces.

Graphene for example is made up of hexagonal lattices or layers. Each layer has intermolecular forces with the layer above and below it. Though the individual layer of graphene itself is made up covalent bonds. (where intermolecular forces exist but play a negligible role in the structure).


I hope that clears things up.

I'll reiterate, all molecules have intermolecular forces, as the only thing needed to generate them is an instantaneous dipole and due to probabilistic nature of matter this will tend to be the case.