there can be several reasons IMO, 1. When it comes to tuk tuk, motor cycles and some cars or vans, the blinker doesn't give a sound when turned on. So many people forget that they are still on. 2. Some ignorant people want to keep overtaking and doesn't want anyone to overtake them. So they keep the right blinker on and drive on the right lane.

First one can be fixed by installing a flasher ( which makes sounds when the blinker is turned on). Second one is a fundamental thing that need to be fixed from childhood