The simple and somewhat glib answer is: because teaching is a harder job than many realize.
At this point in the term, I'm only capable of a slightly more complex analysis and that, I think, is part of the answer right there. Teaching is a cyclical profession of intense, focused periods of activity, during which times many routine tasks are pushed to one side. We often think we'll get these done during those famous teachers' holidays. However, many teachers spend a good proportion of those holidays sick as, exhausted, they crumple once out of the classroom spotlight. In many ways, getting through a half term can feel like sprinting a marathon.
Add to this the mental stress of 'keeping control' in a classroom: both of the pupils and of oneself, and the improbability of taking a real break during the day (break duty, detentions, setting up the next lesson) and, really, why are we even asking this question?
I think the loss of new teachers is at least partially down to the relentless nature of the job. Perhaps, in the PGCE year, and in the NQT year, people expect to be busy and tired and stressed. But after a couple more years, when the pressure isn't easing, maybe that's when it finally dawns that it's not about being new at it; it's not about getting used to the curriculum requirements and the latest teaching ideas; it's not about inexperience making things harder: it's just hard. And there comes a point when accepting that your working life will be hard forever is just a step too far.