Serious question: Why don’t they leave? I’m not saying that they should, or that conditions shouldn’t improve, but if academia is that stressful to you, why stick with it? The only potential outcomes sticking with it are that you will fail later on, or you will fill a role with an increasing amount of stressors.
It's also important to note that people who want stay in academia tend to do it because they love research. It doesn't pay more, it's harder, it's more unstable... So a lot of them will enter knowing already they're going to be facing a lot of difficulty and stress.