A colleague of mine, Khalid Khan, recently wrote how many people run with poor postural alignment, and how they should pay more attention to this. On the surface, he is correct, and to the extent that people notice bad habits they should do what they can to correct them.
On the other hand, most people have no idea if, or how much, their body is out of alignment. We tend to compensate unconsciously for imbalances that occur for any of a variety of reasons, including accidents, internal disorders, congenital conditions, poor postural habits and adaptation to incorrect ergonomics at work, etc. As someone who has undergone chiropractic care after minor auto accidents, it's amazing to me how hard the body fights to regain that sense of equilibrium adapted to misalignment, rather than simply re-establishing conformity to proper realignment.
I agree it is often obvious when watching runners that they have alignment problems, but this is not simply a matter of willful disregard of proper form. For instance, someone with a bad back may feel less pain working around the area of injury, resulting in a lower shoulder, rotated hip, etc. Attempting to straighten up will feel unnatural. Eventually this person may wind up with a sore knee, but that is a symptom, not a cause.
Another consideration is shoes. Poorly designed or mis-fitting shoes can cause problems all on their own. Smart runners will take the time and spend the money for the shoe that best fits their foot, but that can be both time consuming and expensive, especially nowadays.
Even the best runners have idiosyncrasies in their movement, and trying to correct that does not necessarily improve their running. I ran competitively for 5 years (best personal times: 2:00 half mile, 4:24 mile, 9:45 two mile) and I got to see the postures of a lot of faster runners. Some things can be corrected, such as learning more efficient arm swing and focusing the gaze to minimize head movement. None of these, however, have much effect on basic underlying postural alignment.
Furthermore, attempts to completely balance the body have been known to have negative impacts on competitive athletes. One example was the unexpected impact of rolfing on professional ballet dancers in the 1960's, which ended some promising careers. Even a top athlete like Bruce Lee had one leg that was shorter than the other. We learn to use the body we have, and changing its structure can erase muscle memory and cause proprioceptive conflicts that inhibit best performance.
One problem distance runners have is their leg muscles get tight which shortens them. Stretching helps, but most people don't do more than basic loosening up, and over time they adaptively lose range of motion. This in turn moves stress up the line to affect other body parts, such tight legs impinging on the back. Many people don't know how to stretch correctly anyway. Stretching cold muscles can cause injury, and how many runners understand the value of warming down properly, or are able to even take time to do so if they are running on lunch break or elsewhere in a busy schedule?
So, with all these considerations, how does one simply stop running poorly? A professional athlete may have a support team of a chiropractor, trainer, massage therapist, nutritionist. They may use video of their movement to identify correctable problems. None of these are readily available to most individuals, for whom activities such as running, martial arts or dance are an adjunct to a lifestyle, not a goal unto itself.