Basically because it's a text based game with 10 different sex-poses scenes written on average for each character (counting random encounters) for over 60 characters, some scenes being threesomes, foursomes or even orgies, it's not as easy as telling the fans "draw fan art pls" in this case, because you'd need a coordinated team for that and not wanting to pay for it would simply be kinda shitty and taking advantage of your fanbase... the dev is already putting extra effort and spoiled us by putting ONE scene accompanied with visual, but trying to do it for every pose of every character and program them all in would simply slow the process down massively to make sure no issues were had with any of the additions programation and to make sure the game is not over 10GB of storage.
but most importantly Monster Girl Dreams seems to want to remain text-based and that's good. (sometimes image is simply not needed, and erotic novels without pictures are a thing...Some scenes maybe will look hotter in your mind than in image just by going with description alone). By remaining text based the game can allow itself to have more choices, be easier to be programmed, have more scenes, more characters, weight less in your computer, and the dev can rest knowing that the effort he puts in the writing is appreciated since, lets be real, in games with images people don't pay that much attention to the writing of erotic scenes and also make it more engaging for the player who plays for those scenes, since he/she has to pay attention in order to enjoy it.
Its not that "it hasn't got sex-arts yet" or "it's to hard or there's no money", is that monster girl dreams seems to be designed to be and remain a text based game; same with other games like Trials in Tinted Space or Corruption of Champions. If fans want to make erotic fan art they'll probably upload it to rule34 and that's it, no need to implement it in the game if the game doesn't need it, in this case the art is 100% optional, of course he'd say "maybe after the game is done", because if he starts applying it before it ends the game's production could take way to long and he seems to focus on advancing or make additions to the game as a story or mechanically before he focuses on the optional features.
Maybe after he is done he can see the completed game with all the completed scenes and say "ok, this scene could use one image, this one is not that important", but until he has completed it i doubt he could judge fairly on which scenes are deserving and which aren't.
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