The obvious answer to this question may be that researchers see themselves as academics in training. Another one is that research alone is seen as a career generally has a limited life span and lecturing is a way of keeping research going. There is some really interesting debate on this on the Vitae research staff blog www.vitae.ac.uk/rsblog.

But the reality is that, depending on you field of research, the chances of you becoming a tenured academic are quite small. http://www.vitae.ac.uk/researchers/156431-249071/The-Ninety-Per-Cent-factor-Part-1-Getting-in-the-ten-per-cent.html

One idea is that the focus on academic careers is because this is the predominant role model researchers have. The people you're most likely to observe at work and think of 'oooh, I'd like to be where they are in the future' are likely to be academics. This is really where career stories come in: finding out that there are really valid alternatives to academic careers where the skills you have spent years honing will be used. www.vitae.ac.uk/docs is a collection of some career stories of researchers. There is also a great new post here: http://www.vitae.ac.uk/researchers/156431-251991/The-Ninety-Per-Cent-factor-Part-2-In-search-of-Plan-B.html

what sort of career do you want? Do you have a plan B?

As it is so competitive to become a lecturer and so many researchers really do want to become one, it would be a real shame to think that people who are not passionate about it are adding to the competition! The perception that academia is an easy career path I'd argue is down to the available role models again. But the moving into other career paths I agree is not easy in itself....

But this doesn't address the point made by Tennie above that there is still a massive attrition rate at each level. I think the issue here is really about whether many researchers are fully aware of the probability that they secure a position.

If only 10% get a tenured position, that is 90% who leave academia or postdoc for a long time. Is this a fact that researchers are really prepared for when they start to approach the end of their PhD and have to make a career decision?

I had a similar discussion with my supervisor and they said that it didn't really matter which way you went. You could equally well come back to academia but your work had to always be as relevant to your new research question as possible.