In most cases it is because they simply don't have the option of continuing. However, now it seems some are leaving because of disillusionment.
Doug Natelson has a thoughtful (and depressing) post Long odds: and how we spend our time. He discusses how he is currently writing three grant proposals that have an approximately 5 per cent chance of success, and raises the question of whether this is really a good use of his time? I would clearly say no. But he and many people in the US system are trapped. Personally I don't apply for anything with a success rate of less than 10 per cent. But in Australia we have the "luxury" that there are programs with a success rate of slightly less than 20 per cent.
But, the main point of this post is to highlight two comments on Doug's post from postdocs who just left academic science because of disillusionment. I think their concerns are justified (and alarming) and I have no answer for them.
This is why (or rather one of the reasons) why I recently left my postdoc in physics and went to industry. I saw how my friends who were PIs would talk about how they never had time to think about new ideas because they were so busy chasing grants (when they weren't advising students, teaching, do administrative work, etc.), and I didn't want that sort of life. You recently talked about sensationalizing in the scientific popular media, but these long odds for grants also of course cause grant-writers to sensationalize the impact and benefits of their own research. This is generally considered "normal" and "necessary", but I believe it's as scientifically dishonest as fabricating data. Scientists are not only the judges of what's scientifically correct or not, but they should also be the judges of what's scientifically important (because who else can judge this besides them?). This latter function of scientists has almost completely been destroyed in my opinion over the course of my time in science with the adoption of business-like ethics that academia has increasingly taken on. I don't know if this new "ethics" is caused by the funding shortfall or whether it is related to a general trend in society of caring less about one's responsibility to society and more about how to advance one's own interests at society's expense, but it's problematic nonetheless. Science needs to be drastically reformed.The second commenter writes.
I agree completely with the anonymous postdoc from above. I also was a postdoc who left academia very recently. On paper, I think I have good enough credentials to get a decent tenure track position (prestigious groups, high h-index, etc.), but instead, I am also leaving academia for the same reasons as Anon1 mentioned. It has become increasingly obvious to me over the last several years that if I stay in academia, I probably will never do any actual good research. I think I have some good ideas, but I just do not think I could ever work well or have the ability to pursue non-safe ideas within the current funding system/climate. And if I do not think that I will be able to do good novel research, what is the point of going into academia? I see both the pursuit of ideas that could have a high chance of failure and teaching students as the goals and benefits of an academic career. But increasingly it seems that playing it safe and spending every waking hour pursuing grant money is what is expected instead, especially when applying to grants with such long odds. I certainly don't think of myself as some sad story of a great researcher who would have changed the world. Quite the contrary - I am just a normal person, and I am also happy with my choice to leave. But I do think that if Anon1 above and I both came to the same conclusion, then there probably is some other person out there who really did have the next great idea, but will leave because our academic research system is changing for the worse. And that is sad.There is also another sad consequence. Those who remain in academia will tend be those who are content not doing much real science, but are more interested in self-promotion, quite comfortable hyping their research and using students as cheap labour for paper factories. It is just like how some claim university administration tends to attract people who are more interested in power and high salaries rather than in actual teaching, research, and community service.
I welcome comments, particularly from those who have, or are considering, leaving academic science for similar reasons.