Looking back seems so simple, especially in contrast to the effort of running, that concerted full body act of moving as smoothly and quickly as possible for a set distance. Rotating the neck and averting the eyes backwards for a second seems like a trivial task in comparison. There are a couple reasons why this is not the case, and that behavior is not recommended.
The first reason why it is not a good idea to look back is mental. The act shows nervousness about the competition and a possibility of flagging energy. It may be seen as a sign for the person in second to push and overtake. Racing is a mental battle as well, and appearing tired or weak will only serve to spur on the opposition. While it is useful to know where the competition is, possibly boosting their morale generally is not worth the risk.
The second reason is physical. That brief and minor change in orientation has the chance of ruining a race. Everything, from stride to breathing pattern, is altered. Hardly a factor when there is a large gap, but if the challenger is close behind, that gives them a definite advantage. There is also the risk of injury, from not seeing an obstacle, or stumbling while getting reoriented.
Of course, everyone is free to run and race as they wish. What works for one person may not work for another. That is the beauty of running, and life in general. Even when involved in a similar task we have different ways of going about it. But when it comes to race day, on the off chance that I find myself in the lead, I am fighting the demons. Am I faster than the next guy? Am I stronger? Dare I look? I try to remember that it does not matter who else was at the line, and on the course behind me. I can only do the best I can, and perform as well as I have trained. So I push as hard as I can, and run through the finish.
Never Look Back.