At the end of the day, it boils down to two reasons:
Academic jobs are very specialised. You need a very specific experience to even start a project and then you often spend up to a year to really get up to speed. Moreover, PhD students (and many postdocs) additionally need somebody similarly specialised to supervise them.
Academia projects often have a long duration with a pay-off at the end: You have to get familiar with a specialised field of research (see above), plan and conduct a complex piece of research, write a paper on it, have this paper go through an (often long) peer review, often do some more work in light of the review, have it reviewed again, and published. And only after publication it is a serious contribution to your academic CV.
Something similar holds for PhD degrees: You have to finish it to be worth anything. Two half PhDs don’t make for a full one.
As a result, you cannot simply switch jobs: A specialised job market is small. While this affects supply and demand equally, you almost certainly have to move when switching jobs and it may take some time till a fitting job pops up. Many people in academia start looking for their next jobs years before their old contract runs out for this reason. Also, when you have not just completed a project, you wasted quite some time from a CV perspective. Finally, your potential employers (i.e., professors) are aware of the long time it takes to get you up to speed and are understandably very hesitant to hire somebody who may not complete the project.
Thus, leaving an academic position may very well be a move that ends your career (at least within academia) and may also lead to regret of not finishing a project on the long run. Therefore, many academics are more willing to tolerate bad conditions – not that this excuses them.
Finally, a few words on your observations regarding advice given on this site:
Many comments and answers always try to play devil's advocate, for example:
- trying to find a reason why a supervisor may only seem abusive but are actually just doing their job (and maybe their character is just not very friendly),
We had several situations in the past where it turned out that a complaining student severely misjudged a situation or even behaved blatantly misbehaved themselves (and just consider how often we never get to know these things). Of course there is a fine line to be walked between advising reasonable caution and victim blaming here.
excusing rude behavior such as harsh criticism with no constructive feedback simply because it's common place in academia
Non-constructive feedback is inexcusable in almost any situation, but putting a high scrutiny on somebody’s work is indeed necessarily common in academia.
Both my personal experience as well as some questions we received show that some people have problems with this, no matter how you phrase the feedback.
often proposing to give bad situations two, three, and more chances because giving up is shown as a lack of desire to grow in character (often for no reason but just "because it's bad to give up on a situation").
I would not phrase it like this, but as reasoned above giving up is generally a bad career move in academia. For example, enduring a few more months with a moderately bad supervisor to finish a PhD is usually sound advice. Of course, at the end, the student needs to decide the lesser evil for themself, but it would be negligent not to mention the career consequences.
Also, no supervisor is perfect and given the strong individual dependence on supervisors in academia, supervisees will get to feel the imperfections more strongly (same applies for the good sides). This is different in most industry situations, where you have lesser dependences on more people.