While many might have come to creating their art in a non-NSFW-context first, you can't do any sort of erotic content if you're not into it. It just doesn't work. I can create any form of SFW content without my heart being behind it, but porn and such will just not be appealing without a tad of your own feelings.
So "that's where the money was" cannot be the full story.
I don't do hentai per se, I do 3d renders. Yet, I started out with 3d stuff specifically because it was a way to bring fantasies to live. I do my art for the same reason many people play adult video games: It's my way of enjoying digital art in an interactive way, interacting with the things my brain can come up with that cannot (and/or should not) ever be done in a real porn setting with actors. It's fun to express even those sorts of fantasies that would have been rightfully banned to the back of my head otherwise.
I always had a passing interest in 3d art and such, yet it was the NSFW-part that really kept and keeps me going most of the time.
Writing short stories was always a hobby of mine, so I kind of fused the two and started doing comics.