Allow me to explain.
If you work at McDonald's and it sucks, chances are there's a Burger King, Wendy's, supermarket, etc. that pays around the same near you. All of these jobs will probably suck and not pay well, but losing your McDonald's job doesn't mean you're out of job offers. In fact, you're probably more employable than most people if you have a clean record.
However, let's say you went to college for a very specialized field. This field is located in only a few places in the country that you have to geographically be in.
Now let's say this job pays you better than any job in the area. It also gives you great benefits that other companies won't.
Now let's say you're promoted to a senior level in this company. You're now making a specialized amount of money in a specialized field in a very specialized role that probably doesn't exist in many places in the country or even the world. If you lose this job/quit/are fired, you can't just walk down the street and get the same type of work with the same salary. This job may even have skills that are non-transferable to other fields if you want to make a career change. But let's face it, if you change careers and start from the bottom you'll probably be making shit money and have to work your way back up.
Likewise, this job probably affords you a certain lifestyle. You don't have to spend your money, but you can afford a nicer house, a nicer car, etc. It's really hard to go backwards in terms of lifestyle, so chances are you'll do anything you can to hold on to it.
So many high-earners I know are just shells of people. I can't discern any personality or interests from them, most of them just talk about their jobs. They're constantly thinking about work 24/7 because it affords them this lifestyle and they're probably fucked if they lose it, especially if they don't own the company or have any equity in it.
There's your downer note for the day.