It's all about the gameplay - if the game is fun, you'll keep coming back for more. That's really it.
I always used to say that it's all about the gameplay and story, that the gameplay is the fun part and story is what keeps you going and coming back, but in times of Youtube where you can watch all the cut-scenes and dialogues, it just doesn't hold true anymore. I came to this realization literally last week when I was finishing my 4th walkthrough of The Ascent, because I keep coming back for more simply because I'm having ton of fun by just walking around the world and blowing shit up, the guns, the skills, explosives, augmentations etc. just feel amazing and it's just one hell of a fun game to play, hence it's my go-to title when I'm not sure what I want to play, so much that I already replayed it three times. The story doesn't matter, nor do the characters or the world, the gameplay is fun and that's why I keep coming back.
But frankly, Youtube is where I "finish" most modern games nowadays, I just play them until the honeymoon is over, which is a couple of hours usually, and then I go to YT to see how it all ended, if I'm interested in the plot in the first place that is. Because I can already tell after mere 5-8h that I'm not having fun at all, so why on earth would I want to waste 20-80 more hours (open-world FTW!) when instead I can have all the plot wrapped up in a 20-40min. video?
Same goes for replaying old games that simply didn't age well, whether the graphic- or gameplay-wise if we're at it, you can go back just to the story on YT. For example I tried to replay Max Payne 1/2 about two years ago and I simply couldn't do it, the bullet-time is nowhere near as fun as it used to be 20 years ago, let alone the visuals/AI/animations, but that dark noir graphic novel? Still fucking amazing. Starcraft? Same deal, the story told there is still one of the greatest ever, but when it comes to gameplay alone I prefer something like C&C RA2 or Generals really.
But if the gameplay is fun, you keep playing the game over and over again. And it's especially noticeable in MP, the so-called "last match syndrome", where you promise yourself this is honestly the last round, only to spend another 2-3h on the game, and no progression system, collectibles and unlockables can ever give you that, it has to be the gameplay. And MP is actually the best test of the game's gameplay - there are no cut-scenes, no scripts, no dialogues, no nothing to cover up the mediocre gameplay, if it's not fun you won't want to play it no matter how great everything else is.
The problem with modern games is not that there's no replay value per se, it was really never the case if you look back 10/20/30 years, the problem is that most devs don't put the gameplay in the absolute first place, the games are being made by some Hollywood-wannabe directors, the games are desperately trying to be something else, something more, completely forgetting what it's all about in the process. Just think about it - how many games market themselves with the actual gameplay? Exactly. It's all just cinematics, cut-scenes or CGI even, with a "not actual gameplay" label, this is the source of all the issues with modern gaming. But if you take that away? The gameplay is very shallow, basic, mediocre.
If you played Uncharted over a decade ago you already played all the platforming sections in all modern TPP games, it's all the same copy-paste animations and mechanics, so in a sense you are replaying the same stuff over and over again. And speaking of, most games nowadays are TPP exactly, for the sole purpose of storytelling and this whole movie-wannabe aspirations, barely anyone puts the actual player in the center of the events (FPP), or some other perspective, the player is just a passive audience. And many games are melee-based, nobody bothers making any fun shooting mechanics anymore, while those melee mechanics are filled with not even QTE but a single-button finishers, so again, you're not playing the game much as you're just watching it.
And on top of that there's that awful open-world craze going on, where you spend >90% of the game on literally just walking from A to B, now how the hell is walking suppose to be fun? That's the point, it's not, it's boring as fuck, hence nobody wants to replay those games after already putting 40, 60, 80h into them for just a 3-4h story, people actually feel relief that its finally over and they don't have to go through it ever again.
But then again, people vote with their wallet, if that's what sells the most, that's exactly what people will keep getting. But luckily the video gaming industry has a broad variety of content, you just need to be willing to look at it, if you close yourself just onto the AAA market, that's what you'll get, until a new trend shows up.