Then what's the difference between PORN and these games them ....
Well, there is different variations of porn as well. Since the spread of internet and the adult industry with it. What have vanished from main stream porn just about, is porn movies with story though (regardless how cheesy they was). It became to costly to produce. Now it's more or less straight to the point and add as much shock value as possible to deliver on one extreme after the other in hope of stay ahead of the curve in a cut throat business.
Creators of these type of games kinda stuck between a rock and a hard place. If they try something different, they get yelled at, and how dare they don't include sex and lewdness in first release!
Some creators look at what is popular (and who gain support), and try deliver on something similar(. But so do just about any entertainment industry/business. People create clones of candy crush, and shit like in hope of become the next big hit. Or all these crap battle royal games and so forth. How many Battlefields or Call of Duty will we get? They find a comfort zone and stick to what sells. Funny thing with main stream game industry, its within the indy devs circles you find new ideas and games that try to deliver on something new and fresh.
A creator of adult games can be good at make renders, poses, code and all that stuff, but if they suck at writing and make story, chance is they will never become more than mediocre at best.