The answer to that is that it has become much easier and more common than it used to be.
Many people just do it - without any help - they just seem able to take the leap.
But this may be a subset of people who are less risk averse or more creative when it comes to having ideas for moving on from one job or one career even to another.
Last night at a social event I spoke to someone who had started out as a tax inspector, then been a teacher and now is a restaurant owner. No one helped her make these changes and although she agreed she could have done with some help here and there - she just got on with it.
I am not saying everyone should do this nor does everyone's circumstance facilitate such radical change. However it is an example of someone who has had at least three main career routes in a lifetime and who knows there may be more to come..
I reckon most people could do this if they were empowered to do so and had a modicum of support and guidelines.
Its often a matter of knowing where to start and equally what not to do or what to stop doing.