Why do you need an "Engine" for wallpapers in the first place?

So you can have an animated or interactive "wallpaper". The malicious wallpapers in the OP are hentai games.

I use them because they look nice and with an OLED I wanted a dynamic wallpaper so I wouldn’t get screen burn in. There is adult stuff but everything I use is just video games and matrix scrolling patterns, that kind of stuff, so I can tell my monitor is on without getting burn in. I’ve got a high end desktop so can take the minor performance hit running it all the time.

Unfortunately seems like I’m going to be uninstalling it.

The 'wallpapers' in question are pirated games made in renpy (python game engine) or rpgmaker (js based), which makes them a really good vector for malware. As another commenter noted this is a bizarrely common way for Chinese people to get porn through the great firewall.

Opensuse (pre 11 iirc) used to have a really cool background where the lighting changed throughout the day, that probably used an engine of some sort.

Because they are not static images. That's the whole gimmick

Because it's one of the only ways to get porn in China