They've probably correctly surmised that outside of a small handful of whales like Runey the amount they'd make off of licensing their games to be used as a professional graphics engine wouldn't be worth the legal fees and liability. Virtually no devs in Japan use KK or any of the HS variants for AVNs which as a Japanese publisher would be their first concern. A small handful of Chinese devs do but professionally licensing porn software there is laughable. Most western devs aren't even pulling in 6 figures annually. You can't get blood from a stone as they say.
On top of that it would increase the start up cost of using an Illusion engine for your porn game graphics which would just choke out the indie industry and push more devs to use Daz, Blender or 2d art. Biggest draw to Illusion as a first time dev is that it runs on a potato and you don't need to pay expensive licensing fees or spend out the ass to purchase custom assets. Increasing the cost at this point would just decrease the demand.
