all elements except selenium form dichlorides and dibromides. why?

Not all the chalcogen [oxygen family elements] can form dichloride and dibromide. Only oxygen and sulfur occur as dichlorides and dibromide. Beyond sulfur as we move down the group none of the element occur as dibromide and dichoride. Oxygen and sulfur with halogen form co-valent bond and we know strength of co-valent bond decreases as the distance between the nucleus and the shared atom increases.?
Due to larger size selenium fails to form effective co-valent bond and is unstable. Though selenium occur as selenium mono chloride Se2Cl2. Its a inorganic compound.