Why do covalently bound substances have low melting and boiling points?
Most do, some don't.
Both carbon (graphite, and diamond) and silicon dioxide have exceptionally high melting points, and these materials certainly feature covalent bonding. They owe this property to their molecularity. Diamond, for instance, is an infinite array of carbon-carbon linkages which are not constrained to molecular boundaries.
Molecular substances tend to have much lower melting and boiling points because the forces of intermolecular attraction tend to be weak to moderate. On the other hand, ionic solids are non-molecular, and each positive ion is electrostatically attracted to every other negative ion in the lattice. (Of course, each positive ion is also electrostatically repelled by every other positive ion in the lattice, but if you sum up attractive and repulsive forces, which you can certainly do, the net interactive force is positive.)