It explains perhaps my opinion that running can be dangerous. However, my opinion isn't ill-informed.
Let's say you're running at 5mph on concrete and you trip on something. Amazingly, of all the anecdotal evidence I collected about falls while running, almost all of them were simple scrapes and bruises. A couple were sprains or knee injuries, and virtually none of the collar bone breaks I expected when people try to stop themselves with their arms. A few people hit their heads pretty hard, though.
There's also more self-inflicted injuries that are common for runners, such as Runner's Knee, tendonitis, hamstring pull, plantar fasciitis, shinsplints, ITBS, lower back pain, and stress fractures. And while all these things can happen to non-runners, they are much more common with people who run frequently or for long distances.
People with problems such as hyperextending knees or other pre-existing conditions are often encouraged to try exercise which is easy on the knees, by doctors. But the repeated impact of running might not be what's causing the pain in their knees when they run. It might be knee fairies.
And while knee osteoarthritis hasn't been found to be significantly higher in runners vs non-runners, hip osteoarthritis has. It's commonly held now that the lower BMI of runners contributes more to their lack of increased osteoarthritis, rather than the assumption that running doesn't have a negative effect on the knees. Also, mysteriously, doctors recommend those over 45 to substitute some of their running time for swimming, cycling, and other less knee-impact-prone activities. But maybe old people just get bored quickly or something.
It is also understood that trying to run before you're ready can result in injury, much like trying to weightlift a lot before you're ready can. But there are no guidelines about how to know what you're ready for, and certainly no warnings anywhere i've ever seen about running before you're ready. In addition, people who are encouraged to jog or run as part of a getting-fit program are at higher risk of injury due to a lack of muscles supporting the joints and bones, and adding additional stress to the knees.
Alternative exercises avoid these common issues by providing a structure from which to safely begin and increase progress, provide the same health benefits, remove almost all of the potential for common running-related injury, and don't involve spending lots of money on running gear, not to mention take less time and are more convenient.
But I guess I just don't know what i'm talking about.