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.
You understand it. It is human nature to strive for improvement, any consolation or comfort for being lazy and failing to make progress is propaganda aimed to weaken you. Hundreds of millions of people are stuck in the same jobs their whole life. Are any of them happy? No, with a few rare exceptions. There's a reason why retail workers or government clerks are often miserable, and it isn't just that the customers are terrible people.

Not entirely true, it has a deeper meaning than just for the job. It is all about having peace of mind and not being consumed by greed. The statement does not conflict with having ambition and desire to improve oneself, and it only conveys being thankful for the present and do not be frustrated if plans failed.
You've spent too much time reading images with quotes on Instagram.
Being content with your shitty job just means that you have your priorities backwards and are probably leading a bad life.
Few people have a job that is good enough to warrant feeling content with it.