-
I always stop playing about 20%-30% of the way through as they're just all so samey.
All?
The first one was very thin and repetetive; even most committed OG creed fans admit that. If it hadn't been for the crazy SF conspiary-theory satire that framed it, I doubt anybody would have bothered sticking with it at all. The second one was a huge step up in every way, and added a fair amount of side content, but the next two were about the same Italian guy doing more or less the same thing (they were supposed to be DLCs but grew big), with only incremental changes to gameplay. So if you weren't sold on Super Ezio, it would get tired before his third outing. If you stopped there, fair comment.
By the time of Black Flag, though, they'd reached GTA San Andreas levels of things to do besides stealing cars - I mean, stabbing people in the back - which has kept on increasing as the series progressed. The tone and balance became so varied from game to game, some of them requiring almost no stabbing at all while often making you wait 20 hours for your secret stabby knife that you weren't going to use much, that the player base became hugely factional. There were endless arguments about what makes a "real" AC game, and whether the latest one qualified, even before the three huge RPGs that mostly tried to be as unstabby as possible. If they're really that samey, somebody needs to have a word with the fans.
They're boring
Aye, well, there's a clutch of committed Dredge fans on here and others who find it boring. So it goes.
There are definitely a lot of games where what I've just described is bad design and punishing grind. But in Subnautica it's purposely part of the game experience and fits in. It's a survival game where progressing from teetering on the brink of death to building the technology to let you build the technology to escape the planet is the point. You're not just tediously searching the wilderness for loot, you're finding and exploring lots of creepy wreckage for both tech components and documents that explain why and how your ship crashed. They did a pretty good job of sprinkling the exploration and survival with surprises and rewards. You also know very little about what the main story is till you go look, cause it's a mystery/survival/exploration game.
So I think your point is valid about a lot of games but not so much with this one. That said, there's grind and there's core game activity that some people don't like, so they call it grind. Which it is, to them, but they should have bought a different game. Some people said Assassin's Creed Black Flag was a grindy game, because they're weirdos who don't like being a pirate.