Because my dick goes limp while spending an hour unlocking sex scenes with distracting puzzles to solve and shit then when I get to a scene I'm aggravated and no longer in the mood. I used to keep up with a like 4-5 sandboxes (mostly for the story, not really for fapping for the previous stated reason), then gave up when the code was changed that broke saves on all of them. No skip dialogue option because it's a sandbox = have to jump through the 1000 hoops again to reach new content = fuck that, I'll just wait a few years and try again when it's completed, assuming I haven't completely forgotten about it by then.
