Why do simple molecular substances have weak intermolecular forces and why do giant covalent substances have strong intermolecular forces? Why do simple molecular substances have weak intermolecular forces and why do giant covalent substances have strong intermolecular forces?

I understand that in simple molecular substances the atoms within a molecule are held together by strong covalent bonds, but the intermolecular forces between molecules are weak. 

But I don't see what makes giant covalent substances have stronger intermolecular forces? What makes them stronger and have higher melting points? 

I thought I understood but I think I must have gone wrong somewhere and now I am very confused. Help would be much appreciated. Also please explain simply, chemistry does not come naturally to me so I might get even more confused.
In simple molecules the intermolecular forces are, in order of increasing strength, dispersion forces, permanent dipole interactions and hydrogen bonding (which has significant covalent character but is generally considered to be an intermolecular force).

By contrast giant covalent repeating structures such as diamond and $\ce{SiO2}$ are not molecular in the same sense as they can theoretically be infinitely large. Therefore they do not really have intermolecular forces but they are simply held together by covalent bonds between the atoms in the structure.