If someone thinks that he/she is developing retinal detachment, why should they first contact the ophthalmologist or vitreo-retinal specialist rather than call 911 to go immediately to the emergency room for retinal treatment? isn't this an emergency?

Hospital ER does not treat retina detachment and ER won't admit you for this urgent eye problem. Furthermore, ER doctor is not trained for diagnosing the retina disease and even a general ophthalmologist may miss the diagnosis because special equipment are required for exam even if one is called in. So the answer is No 911 for suspected retinal detachment.

If you do have a true retinal detachment, this needs urgent attention, which would not be available in an urgent care center or hospital ER, but personnel would likely be available to assist your contact with an appropriate ophthalmologist. A direct visit would be unlikely to be productive.