Note: I'm not a lawyer, just someone with some knowledge regarding Law, this is not legal advice.
Some porn sites give away sample images, and I'm honestly not sure to what extent those are legally protected.
If they are free samples gave to sites owners, you have the right to use them, but not to distribute them, and the obligation to point to the owner site. Yet generally this authorization is limited in use, what mean that you can present them as a sample for this set/site, not in the way they are used by games.
A lot of the gifs are likely taken from previews of videos, as well. So probably a combination of porn games as a whole being too niche for Mindgeek to care about legal threats, and patreon somewhat serving to launder the whole process by being donations for a continued service.
It's mostly this. Trials aren't free, both in terms of money and in terms of time.
The only reason to starts one against an indie creator who earn near to nothing is to send a message ; "don't mess with our Intellectual Property". A message that will not become effective before you win the trial, therefore generally not before one year, and that will cost you more than the defendant will be able to pay you during his whole life.
While, obviously, Mindgeek would prefer to own and control the whole market, it works a bit like this forum. The more people will see your content/game, the more likely you are that some decide to spend money in order to see more. Therefore, starting trials would be counter productive for them. Globally speaking, they would spend money in order to earn less money.
But don't dream, if one day someone using their IP starts to have 5 digits earnings/month, they go after him. You can mess with their IP, but there's still limits to what they are ready to tolerate.