What did you think the F stood for in F95? It's almost like a requirement, and you end up not being being any more surprised than, naturally, you would be finding transformation games on the TFgames site. Of course, you question whether you should look for more games that don't have incest so that your brain isn't trained to expect incest from nearly every story. It's easy to derive an instant sense of comfort from the "quaint" family dynamic these games rely on, but I guess it could be troubling that these are very artificial setups. Some of these stories should play out the moral anguish, an inevitable sense of dread from being caught or some consequences like having an authority figure exile you for your despicable actions.
In any case, I am not sure why it's not a more controversial fetish except that it somehow doesn't overly offend the sensibilities of most people that visit this site. No matter how it is portrayed in games, incest is often a heinous and illegal act that can bring the wrong kind of attention to the adult games community, and that is why it is sometimes banned. It's hard to point to any kind of movement behind the prevalence of incest because it also seemed to surface out of nowhere then almost dominate free (real-D) porn sites over the past 15 years or more. Additionally, Game of Thrones was a popular show that subjected the general populace to both explicit incest and shameless child murder, and, in doing so, shattered classic, golden rules of mainstream TV.