Yes, it's quite a normal phenomenon for most more and more runners wearing glasses. There is no doubt that making them look cool is one of the reasons. Bu t the most reason is shielding their eyes while running. As we know, excessive UV rays can contribute to cataracts. So, It is good to wear sunglasses to shield the eyes. Besides, there are also particles and flying insects in the air, wearing sunglasses while running can protect their eyes well. So, in order to not worry about the damage while running is the essential reason why they always wear sunglasses.
As far as i know, not all runner wear sunglasses. Some of them wearing sunglasses not just to be cool, but protetc their eyes and provide them better vision. You know, sometimes, runner run in the sun. And the sunlight is really boring to shine that affect them focus on the road. Wearing sunglasses can reduce such intensive sunlight thus give them better vision. Besides, when they run, the qucik speed can create wind and partcles in the air. Wearing sunglasses can prevent those tiny things from damaging to eyes. From this point, wearing sunglasses make them running easier.
All right, that is a good question I think. So, as a matter of fact, not all players or runner wear sunglasses. And the primary reason why they choose to get a pair of sunglasses is that they need to block those shining sun light, which is annoying on the court, also, some of them wear sunglasses in order to be cool or shield from wind, for they are running at a high speed.
The glare from the sun can get pretty bright. If you do not want to be blinded and run out into traffic if it's sunny day. You need to get a pair of sunglasses for running. You will find that it is much more comfortable to run without the glare. (I'm not a good runner, so all comfort is welcome).