Why Brahma, Vishnu and Shiva are not three independent deities.........?

They are three independent deities. Brahma represents Rajas (Activity & Creative energies), Vishnu represent Satva (Protection & Preservation..it also represents 'absolute emptiness' in certain beliefs) and Shiva represents Tamas (Darkness & Destructive energies). So, in many ways they are different and there is no way these energies can exist together. Otherwise, the people who wrote down theories and fundamentals on these deities would have made them stay together. In yet another unfortunate twist, Brahma has been downgraded to a mere person who has creative abilities, but not a god to be worshiped. Shaivaites and Vaishnavites have glorified Shiva and Vishnu to a great extent, but ignored Brahma who is the creator of everything. I always felt that Brahma is a very important deity because of his creative and active energies. How would any gods create avatars or weapons without creative energies?