Because I do not want to risk SMIDSYing a cyclist/motorcyclist (especially at night)
Also, especially on busy mini roundabouts, It is important to actually look at the other traffic to be sure of what their intentions are.
One near me (where I frequently have to make right turns) is plagued by people flying in from the left and paying no attention to my right turn indicator because they have simply glanced rather than actually looked.
And the indicators on my old Landcruiser are a good 10 square inches of yellow lens, not a poxy little coloured bulb hiding inside an otherwise clear lens like on many modern cars, so there is no excuse for missing them
(I have become quite adept at spotting the ones that are not going to stop and stopping myself with my Bull Bars 6" away from the errant drivers door as they go past!)
Also, remember, Older people are likly to have poorer eyesight. Still well within the legal requirements, but perhaps not as good as when they were younger. So yes, shock horror as people recognise their limitations and are careful about ensuring that they drive within them
Oh, and to add, Also re mini roundabouts. I wish to approach slow enough so the people to my left have a chance to see that I am there and have an opportunity to give way. Far to many people simply glance right and say to themselves "Ooo I can go" giving no thought to the visibility for and ability to react of traffic joining from the left.