"Guro hentai movies" have never really existed. Lots of anime produced in the 80s and 90s had ero-guro influences, sure, but there weren't many, or any, anime films made to be used solely as guro fapflicks. I think what you're referring to are barely censored, over the top OVAs that were pretty common place around the time of the Japanese economic bubble. You're right about there not being many guro scenes in Japanese animated shows/films these days, but hey, that's the way it is. There are tons of animated guro scenes that take place in older anime series/OVAs that I'm sure you've never seen before. There's enough old guro to satiate your lust until newer scenes come out (and yes, there are still guro scenes in anime).
https://www.sakugabooru.com/data/3a8f53bea69272998b06cf48cc29229f.mp4
https://www.sakugabooru.com/data/cc5921fcd12e6a8c9d2477dbf2009e6c.mp4
https://www.sakugabooru.com/data/b5e367fb969747fabfb93f286bc61bbb.mp4
https://youtu.be/ljK0HxkgBrY?t=306
The types of scenes posted above usually had a single, incredibly talented, animator behind them. I'm willing to bet you miss well animated body horror/grotesque scenes, right? Well, most of the sakuga anime scenes done by professional studios usually want their high-end anime talent, animating something that most of the world doesn't find completely putrid and immoral. The reason why there's a lack of "guro hentai movies" is because they're simply not marketable, in an era where people like "clean" anime. What you want is a niche within a niche.