It takes 1 minute from start to finish if you scroll through 580 pages of text. What's the point of erotica?
You seem to have confused a medium / format with a genre.
The thing with a VN format (in this case, for example) is that it takes far too much disk space for what really boils down to a reading and viewing experience of less than 100 pages of texts + twenty-something drawings with faux-animated effects and background music. Contents of a VN may be pornographic or not, just like contents of a plain text file may be pornographic or not. I am not questioning the reason behind erotica, so stay focused.
At its simplest, one could do a slideshow of graphic files or scrolldown of a text file in a viewer and call it a virtual novel. There is no extra entertainment value or fap-factor added really if you compile these graphics, text files and sound effects into an engine which lets you do nothing besides just skipping forward and making four meaningless choices along the way. You're still reading a novel, only 'virtual'! Unlike a real novel, you can't simply go to any page at any point of your reading experience, you have to sit through everything, from virtual page one to virtual page last. Oh, by the way, there are several variants of page last, so you can 'choose your own adventure', kinda-sorta.
