Useful keywords include "contortion(ist)", "flexible", "(human) pretzel" and while broader "gymnast".
Besides already looking unnerving rather than sexy to many people even when performed by real people I imagine 3d renders require a lot of effort to make it realistic. Lots of self-collision means lots of clipping issues, 3d meshes don't deform the same way squishy bodies do and it carries the risk of exposing cut corners or other flaws in the model e.g. low-poly meshes or low-res textures. And all of that before even doing anything explicitly sexual in such a pose.
Basically a lot of effort just to serve a specific niche fetish market, and not something you can easily toggle like "mother<->landlady" either. For comparison think about how few games feature female body hair even though it's natural and usually doesn't require interaction.