Can you back that up with data? I have my doubt.
LinkedIn is full with people ineligible as too old, above 40 usually
Unfortunately I can't, this is just my observation though.
Of course 40 years old is already old, at such age you're supposed to be at least a senior manager or a director. I don't buy someone who saying "oh different person has different fate, you already good enough even you didn't achieve that position". It's all bullshit, 40 years old who can't reach senior manager position are either lazy or not good, people need to accept it.
It is not old. A lot of people use anything and everything as an excuse. If you are in your 20s it is one reason not to learn, if you are in your 30s another, 40s another and so it goes. A lot of very successful people made career changes late in their life. They need to just stop being lazy.
You shouldn't stay in a job you don't wantz you'll find another one, don't listen to employer propaganda that you won't find something better, there's always something better, don't waste your time and life somewhere you don't belong.
Yep.
Just like in my country the minimum salary is $400 per month, but there are job vacancy for full time job, 12 hours work, 1x off day per month and the salary is $200 per month. With such work schedule, when they can learn a new skill?

1 day off per month, so they work almost double the full time every week?

Saturdays, Sundays and any time you can squeeze during the day before and after work is for learning a skill. Learning a career skill is most often not physically demanding, and often not mentally either but rather fulfilling. For lazy people on the other hand it is always demanding.
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.
I think that was the reason why job market is fucked up.
I read many employees complaint about their jobs in social medias, they explain how their jobs aren't worth with the salary because it's far from the contract/job regulation. Guess what people react to their post? Instead of working together to report/fight/cancel the company, most of them said "be grateful with what you have, many people are jobless" etc etc.
It seems most people already adopt slavery mindset.
I agree, they are virtue signalling slaves that don't have any independent or willpower. Sheep can organize better than most humans. Occasionally there is a weak protest here and there but most often it doesn't really accomplish anything. When one does stuff such as accept "nflation-matching wage increase it just shows how weak one is.