That's a propaganda to make people become employee in their whole life because they keep thinking, if they lost this job, they won't get another one.
Most of the time, if you search other jobs when you still have your current job, you can't change your career. So let's say you're a welder, your top job would be an underwater welder. It's because you have no time to learn other skills and you're already tired.
Only brave people can earn much money, like people invest in something, they spend money to get money. People need to have their own "roadmap" instead of worrying to lose their main job.
It’s hard to leave your current job when you have a family to feed, I’m really guilty on this thought. But unless you won’t start taking risk, you won’t land into another, your dream job that pays over the other job. However, this is quite easy to say but actually hard to do when you’re in the real situation. It’s easier to just add another income than to completely quit from your job.