Koikatsu can be perfectly animated the way you want using Koikatsu alone, and you can import your own environments to the studio. Sims, you'd have to make the animations and then go through all the effort of importing, plus you're stuck with Sim locations and Sim artstyle, and you'd have to pause so often just to get stills.
Sims 4 is a bad engine. Simple as that.
Sims 4, unlike Illusion games, doesn't come with already premade studio tool. I don't know if you've actually played Koikatsu or HS but these games come with their own studio tools that allows you to pose, clothe, animate and customize the images. Then render them in full quality.
Where Sims 4 doesn't even have proper camera tools. All you can do is to download some mods and hope you pause the game in right time to take a mediocre screenshot. Then if you really want to make it better, you have to upscale it so it doesn't look like garbage.
Then there's issue of visuals. Sims 4 is an old game. No matter how many mods you cram into it, it won't ever look like modern VN games. Additionally, you are HEAVILY restricted by what Sims 4 allows you to do. All poses, all expressions, all activities are dictated by the game or mods you use. And some modders would be even against using their mod for you to make money off their free work. More so when it's a mod used to make a porn game.