With surveys like that, though it's a self-selecting sample who respond. People who are unhappy tend to want to talk about it; people who are happy don't bother, because they're too busy enjoying themselves.
I don't agree that teaching is inherently stressful more so than any other job. Being in front of children all day is emotionally and intellectually demanding, sure, but it shouldn't be stressful. It should be enjoyable. Teaching a subject you love to kids who you have built a good relationship with and get to see learning and growing over the course of the school year is a joy and a privilege and I certainly experience that joy on a day to day basis.
Teaching is only stressful when the management structures in place within a school allow it to become so. In a school with weak and inexperienced SLT, poor behaviour management systems, overworked and underpaid teachers, and far too high a ratio of inexperienced to experienced teachers - then teaching can be incredibly stressful. However, no more stressful than any other job in which those same issues - weak and inexperienced management, badly distributed workloads, and a majority of inexperienced staff who aren't getting the right support because there aren't enough experienced staff to help them. Teaching sadly is not unique in often providing a poor environment in which to do the job. However, we don't have to stay in schools that are poor environments in which to work, and if you as a teacher are regularly crying, you need to find a new school. Not all schools demand blood, sweat and tears of you. Mine doesn't, and I don't feel stressed by my work pretty much ever. It's demanding, sure, but it's not stressful.
I just get frustrated with the negative press teaching gets and teachers saying oh god it's awful, it's so stressful, I cry every night - it's no wonder we can't get anyone to train. It doesn't have to be that way and actually a lot of us are happy in our jobs. Not all schools are the same. Teaching itself is not the problem - it's how a school is run that provides the stress.
My most stressful job where I often did go to the toilet to cry was actually when I worked in a theatre pre-teaching. There was no money so we all had to do about three people's jobs, I hadn't had the right training to manage the backstage equipment but had to magically figure it all out by myself, and I had to deal with the egos and emotions of directors, producers and actors, all wanting me to fix something NOW otherwise their precious show was going to fall apart, all while a show was literally happening live and if anything went wrong 1000 people would know about it and want a refund. It's making me sweat just thinking about it now! Give me a classroom of teenagers any day, honestly.