Well sandbox games have unique strengths which VNs can never really get away with. They tend to be light handed when it comes to character development so they make a little go a long way with events, they tend to have repeatable events which add to the replay of any given update and there is almost always a focus on a metric shit tonne of vaginas to purse to repeatedly bang (animated in general too). They really are the instant gratification genre of porn games so for long term fans who have put in the grind and now just access several GBs of sex scene after sex scene whilst waiting for updates and when those updates do land? You can often just hyper focus on the content you want with the girl you want which is of course something VNs cannot do.
Sandboxes and VNS can offer the same amount of content an update, same amount of sex scenes and lines of dialogue but fans of Sandboxes tend to be much happier with said content and VN enjoyers who of course have to find they are enduring much deeper story and character development to get to the stuff they want or god forbid they are rejecting love interests and finding indeed updates to be much shorter than they would like because of course they are skipping it. Sandboxes do not usually endorse skipping content, in fact you often find it is mandatory to pursue everything with a pulse to further unlock more content with other girls. I love both dearly but i can certainly see why one would see the same amount of content but less content with one type of game comparative to another.
Obviously the two approaches are very different from each other. But skipping sandbox content you don't want to see is effectively the same as in a VN either cutting off LIs you don't want or Ctrl-skipping through scenes you don't care about. The vast majority of adult sandbox games appear to be made in Ren'Py, which means you can skip just the same way as you do in VNs. Sure, you might have management mechanics, like time of day, money, etc., but not all sandboxes do that type of thing.
Many people act like it's just a matter of format of the game generally, though. However, I've seen plenty of grinding required in non-sandbox games too: tutoring the same student again and again until they are "grateful" enough to remove a single item of clothing; or "accidentally" walking in on someone in the shower, repeated 20 times to lower their inhibitions step by step. The problem isn't actually the game format, but the mechanics the dev chooses to use.
Grinding is generally disliked everywhere, with only a low percentage of people actually enjoying that type of thing. I absolutely hated the constant grind of
Dungeon Siege, for example, even though it was overhead isometric in style like the
Diablo series. There is a right way and a wrong way to implement gameplay, mechanics, and features.
Unfortunately, especially in adult games, it seems devs tend to use sandbox as a reason to add grind mechanics. For example, creating a vast set of locations but then having practically nothing happen in most of those can be a bad experience. For something that should require careful analysis and hunting around (like, say, a crime scene investigation type of game), this is fine: you are, after all, slipping into the shoes of someone who would IRL have to sift through a bunch of locations looking for clues that may or may not be present. But if your goal is only to find a LI at a specific location and time to interact with them, having players hunt is silly; good direction of where / when to go is important in those cases.
So yes, sandboxes can very easily be done poorly. But there are legitimate, good uses of sandbox too.
