Is there an actual mystery here? People have an eye towards getting employment elsewhere, that's why.
It's always a bad look to have been seen as badmouthing a prior employer, even if they deserve the badmouthing. Being publicly gracious improves your employment prospects, or at least doesn't harm them.
Also, many people seem simply incapable of quitting horrible jobs. So it's sorta like being grateful that the bartender cut you off, when you know that you have a serious drinking problem.
That too, yes. I'll never forget when I worked at a major tech firm and hated every minute of it. To the point where I was underperforming and was rightfully being called on the carpet about it. My manager said something like "if you want to continue to work here, then blah blah blah". The words "I don't want to continue to work here" spilled out of my mouth without me even thinking about it.
I was walking on air after that, like a cloud lifted from my entire life. My last day of work there was one of the most joyous workdays I've ever had.
But because my problem wasn't that the company was doing something wrong, but that I was a poor fit into the organization, I didn't have a solid reason to quit and my tendency to power through difficulties overrode everything else. If that meeting hadn't happened, I'd probably still work there and would be hating my life as a result.
It's always a bad look to have been seen as badmouthing a prior employer, even if they deserve the badmouthing. Being publicly gracious improves your employment prospects, or at least doesn't harm them.