The same can be said for manwha authors and animators to an extend as well.
A simplified answer is: Supply & demand. Of course, when you get into the details, the answer is not so simple.
Basically, there are too many aspiring manga authors. When they cannot turn down jobs even if its pay is shit is because, if they do turn such jobs down, there are others who are more than willing to take on such low-paying jobs.
Basically, too much supply. Drawing comic is an easy thing to do for those who have talents for it, and there are apparently too many of such people.
A part of reason for that is people are being exposed to manga from very early age in Japan. A kid enjoys manga and obviously he will want to draw like that. Now, if he has a talent for it, he will draw and will continue to draw. His whole life begins to revolve around manga drawing.
Generally, adults try to get him out of that because they know the reality. However, when does a kid really listen to adults? Not often. I wouldn’t blame the kids, either. They are simply following their passion.
The next thing he realizes: He is in the industry as an aspiring manga artist. He is young. He has energy and burning passion. Most importantly, his style is pretty good.
It doesn’t take long enough for few offers to come in to work for an existing manga author. Pay is shit because he is just starting out.
Okay, fine. He is just starting out. Of course, the pay is going to be shit.
Fast forward 20 years, the pay is still shit.
You can certainly argue about those who are successful. In that case though, I’d like you to remember that there are tens of millions of aspiring manga artists. How many are successful? By success, I mean Naturo level of success. Only those can lead a proper, middle-class, life.
The reality is that you might as well spend most of your earnings to lottery and will likely end up in the same predicament.
When you finish reading a manga, you generally turn to the next stuff. You never really run out of stuff to read unless you are weird enough to have grown specific tastes. The gist is that there is never a shortage of manga to read.
Hundreds of new manga every month, and about the same amount of doujinshi. Now, if you add in even hentai, the amount doubles.
There are simply too much supply. Can it be fixed though?
No. 100% of those in the industry have willingly chosen by their own accord. And, quite frankly, who can blame them? They are merely following their passion. In a sense, these people are better than those robots in university who have simply followed their parents’ demands … although such robots will probably end up better in long term.
The reason I say that is because passion is flame and flame is not eternal. At one point of people’s lives, they will look back unless they die too young.
Those young aspiring manga artists, now in middle age, will look back and will probably long for some form of financial stability.
Meanwhile, those robots in middle age will look back and long for what-ifs if they took their passion to their heart.
Only select few get to have both because life is unfair.
Finally, perhaps watch this video regarding the matter.