Leaving indicators on

I cannot understand why/how anybody leaves their indicators on, e.g. after merging, yet I see it on a daily basis. It's at best annoying and at worst dangerous and makes you scared to overtake them on a motorway or dual carriageway.

So you have a bright light on your dashboard flashing on and off incessantly, and sometimes a clicking or beeping sound, and you don't realise? How on earth does anyone not notice this? People need their eyes/ears testing.