Any input on why that is? Is game dev just simply that complex / time consuming? Or is it because even if they are succesfull, it isn't enough to quit their day jobs?
It's all of this and more.
Since you consider starting your own project, I'll answer from that angle:
Make a game is really time and energy consuming. Especially if it's an adult game, because it will rely on the story way more than any other genre.
You need to come up with the situation. What will happen in that scene, why and how it will happen. Then then keep in mind (or write somewhere, what is better) what happened, because the MC discovering again every now and then that his system like strawberry ice cream is a bit ridiculous. And you also need to keep in mind/write what that scene imply for the rest of the story, because the MC being warmly welcomed to a house he'd been vigorously kicked out few time ago make no sense.
Then you need to write the scene. Therefore the dialog, but also a rough description of the different CGs that will illustrate it. Something that will need to be rewrote, then corrected, more than once, in order for everything to feel natural.
Then you'll need to works on the CGs. For this you need to find a location/background, as well as clothes that match the context. If it happen while MC is at works, you can't place the CGs in a bedroom, and the different people in it can't be dressed too casually, even less only partly dressed. You'll also need to ensure that the expressions and poses match the situation; someone angry can not wear its most beautiful smile by example.
Doing this you'll face some impossibilities, what will force you to rewrite once again the scene.
Next you'll have to code all this. If you limits to basic game mechanism, it will be easy, but if you want more complex ones it can imply weeks of works.
Then finally come the tests. You'll play the scene to see if it carry the expected feeling, going back to the writing or CGs each time it will be needed, what will of course imply a new test. And if it's not a purely kinetic novel, you'll also need to test for all the possible context.
All this, you'll be doing it on your free time, because unless you earn an indecent amount of money, you'll not be stupid and drop your job for something that will perhaps only last two or three years.
And, precisely, you'll do this for few years, what mean that you'll sometimes face a writer block, but also quickly enough want to spend time for yourself and/or with you family. This because, especially at first where the motivation is at its top, you'll do it during all your free time, thinking about your game every time you don't have to think about something else, what will include breaks at works. Something that is nervously exhausting.
So, yeah, making a game, and I mean here making it seriously, is something that take a lot of time.
Then, if you've the chance to be successful and to, more or less, secure a strong base fan, it will take more times. Not just because you works more to not deceive them, but also because you don't want to kill yourself doing it. So you'll decide to only works one day out of two, or to never works during the sunday, or whatever like this that will help you keep a bit of sanity.