I know why he doesn't focus on art. It's because of saturation. Since Super ran Dragon Ball hentai has become less niche and more common. There's a lot of it but as an artist you're not going to be receiving much attention as if you were making a game like this.
And honestly the appeal of Kamesutra IS the fact that you're going around the Dragon Ball world as one of your favorite characters doing a manner of erotic things. There's really nothing like it. Yamamoto's games have that high quality art but there's really nothing mindblowing about the gameplay experience.
I mean in Kamesutra there's a lot of excitement to things like training Chi-Chi every day for 21 days and being rewarded with the awesome sex scenes or playing as Oolong during the photoshoot where you can take choose how much bikini or naked. Stuff like that is what makes Kamesutra so much fun.
In the future we're supposed to be getting stuff like strip clubs, prostitution mini-games, Chi-Chi is supposed to be filming porn movies, and all of the other girls will get strip poker, plus there's all the potential to be had in the bansho spa. You really just can't capture that feel with nothing but art.