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.
What you said isn't wrong. Sometimes we get caught up in words like that, making us afraid to step outside our comfort zone. However, this mindset isn't always understood by many people. Not everyone dares to take the risk of stepping out of their comfort zone, especially if they live in a country where finding work is difficult. So, to survive, the only choice they have to make is to stay in their current job.
In my opinion, if you're no longer happy at your current job or are trying to find another job, unless you've found a new job with an income that (at least) matches your current one, my advice is not to resign. Hang in there and try your best to find a new job.
Alternatively, you should try your best to earn additional income. If there's an opportunity, no matter how small, take it. Take a side job, no matter how small, but don't be embarrassed, and put aside laziness. This way, the more income you earn, the more you can invest, so that even if you're no longer working, you'll have assets for the future.