It is not a job for everyone, but for some reason everyone seems to think they can do it. So you get guys who are full of frustrated feelings of "not belonging" in America, and maybe they don't have a lot of marketable skills to succeed in America, and they become fascinated with Japan for whatever reason, and they hear stories about people going to Japan to teach English. They hear the bar is low, the country is safe, the life is exotic, and people treat foreigners well. It's an irresistible cocktail for some.
So they end up getting hired by one of these ruthless dispatch groups, and moving to Japan, and for a while it is everything they dreamed of. Exotic, fascinating, meeting new people who treat them nicely, or who seem to actually like or respect them. But the honeymoon wears off, and after a while they realize something's not right. They aren't getting treated well. Their pay is barely enough to allow them to survive. After learning a few Japanese phrases, they find the language as daunting as ever. And then reality starts to creep in. But rather than admit that their own shortcomings, they blame their job. They never learn the language, so they aren't able to integrate into their workplace or their neighborhood. Their lack of marketable skills becomes just as burdensome in Japan as it was in their home country, and it causes them to go into a tailspin.
Teaching English to Japanese school kids is a very tough gig, and you need stamina and you need to feel a sense of worth even if you are just reaching one kid out of 30. Very few people are suited to it. If you feel that teaching is your vocation, you probably have a reasonable chance of doing well as a professional teacher in Japan - but without a teaching credential in Japan you may find the economics are just not very attractive. Assistant English Teacher (AET) is the lowest rung on the ladder, and there is a never-ending supply of people who are willing to do the job for minimum wages (or less). There is no job security. Burnout rate is high. Easy to blame any number of things other than one's self: I was overworked. I wasn't being treated well. The pay was low, etc. These are common enough complaints in any company, but in your home country you might expect to use one job as a springboard to another, whereas English teachers in Japan struggle to pull out of the tailspin.