This is the story they're telling.Sometimes weird co-incidences or instance of stupidity in story are frowned upon. For example, in the film Cloverfield, one of the characters only very rarely drops his video camera. Shouldn't he, one might think, put it down and start helping the others? Well yes, but if he had, we wouldn't be following his story. In the film there are literally thousands of people (hundreds of thousands!) trying to escape the city, and many of them may well be filming it, but probably didn't do a good job. We are following the story where the filming was surprisingly good.
While I am fairly intolerant of story stupidity, I will excuse a one off tale like this. Similarly, if the hero just happens to survive the scourge of evil, this is because following those who very predictably DIDNT survive the scourge of evil would be thoroughly dull!
Having said that, while one might get away with it a film, if you follow characters who keep improbably escaping again and again, this excuse no longer works. Dr who no longer has this excuse- being the last timelord his escapes have to make sense, as there are no other stories to be following instead.