The easy answer is: The same reasons that motorists do.
Seriously. There's a segment of the population that is always in a hurry, that prioritizes their own needs above others', that thinks "It won't hurt just this once," or that are just jerks. Those people will exceed the speed limit, fail to yield the right of way, roll through stop signs, or race through red lights, no matter what mode of transportation they are using. They're the road users that make the rest of us shake our heads. Unfortunately, in a land of spotty infrastructure and car-centric transportation networks, this sort of personality is the predominant type you'll see on bicycles, which makes it appear that every cyclist is a jerk.
Then there's the next reason: Inattention. I've run a red light twice in my life-- once in a car and once on a bicycle-- and both times it was because the light had changed and I just didn't notice. This is the highway hypnosis, the concentrating on something else besides driving or cycling, the changing the radio station or seeing what is going on next to you instead of in front of you.
Before I go on to the next reason, let me pause to define my terms. When I say "running" a red light, I mean it in the sense of someone zooming through an intersection with no visible slowing. The previous two types certainly fall into this category, and it usually happens right after the light changes. What many cyclists do, and what causes the ire of drivers to rise, is treat a red light like a stop sign, and is not quite the same as "running" the light. This is generally referred to as an Idaho Stop, because it's been a legal maneuver in Idaho since 1982. It's not legal in most other places, and I don't recommend that anyone do it if it's not legal where you are, because you could be ticketed in addition to making everyone around you angry.
So, why do cyclists do it? Generally, it is for safety and comfort reasons. Suppose you're in a car at a red light and there's a bicycle on your right. While everyone is sitting still, this arrangement works just fine. But when the light turns green, there's a problem--the narrow lane you're in is barely wide enough for a car and a bicycle to be side by side. Since you've seen the cyclist, so you go around him slowly and carefully. The car behind you probably sees him, too, and also proceeds with caution. But what's happening five cars back? The drivers back there don't see a cyclist; they just see that everybody is taking too long getting through the light. And since the cyclist is all the way over at the edge of the road, they may not even notice him by the time they come zooming past, angry at being kept waiting. And angry drivers + invisibility = danger for a cyclist.
|A stoplight in Glasgow.|
Yep, the red & yellow are
lit at the same time before
it goes back to green.
Now, is this a risky move for the cyclist? A bit, if he doesn't look carefully before going. Illegal? Yes, in places that don't allow cyclists to treat red lights as stop signs. Safer? Also yes.
A better idea to begin with, of course, would have been for the cyclist to take the lane before the red light so he wasn't squeezed over to the extreme right to begin with. Doing this also carries some risk with it, unfortunately, because some of the motorists we mentioned earlier will see it and think a cyclist isn't sharing the road as he should. So, if you fall into that category, please be aware of the laws governing cyclists before you get too upset.
So there you have it. Regardless of what you think about cycling and whether the cyclists and pedestrians around you are being pleasant or are being horrible, I repeat my usual plea that drivers be aware and be careful getting around them. If nothing else, getting into an accident is going to put a serious damper on your day.