bond - Why do molecular orbitals in solids merge to bands?

Why do molecular orbitals in solids merge to bands?

For example: In silicon every atom is sp³ hybridised, but when I merge two of these orbitals then it yields a bonding and an antibonding MO. When a third atom binds to one of the $\ce{Si}$ atoms in this configuration, then again we should get two MOs, separated by the same energy. This way we we don't get so called bands. How do the bands form?

I have tried wikipedia and other sites but they answer in brief.