Well, to develop such a service, you need training data, i.e. lots of real child pornography in your possession.
Legality for your viewers will also differ massively around the world, so your target audience may not be very big.
And you probably need investors, which likely have less risky projects to invest into.
Well, and then there's also the factor of some humans just not wanting to work on disgusting, legal grey area stuff.
Yeah, I've also seen it argued on a much smaller scale, that fun in video games is often done on a risk-reward basis. You bring yourself in danger to get a reward. And well, there's other ways than violence to portray that, like spikes in a jump'n'run or a stupid wall in a racing game. There's also other ways to induce fun, like puzzle mechanics. But yeah, ultimately you're left with a small fraction of genres that really work or have been explored...