My experience with these games has been mostly positive. Played It Takes Two and Split Fiction with my partner. I will say, the writing is god-awful, especially in split fiction (which is especially egregious since they're supposed to be writers), but we're there for the gameplay, not the writing.

The gameplay is incredibly broad. We are both technically-minded and have done a little bit of indie game dev, so we were constantly being amazed that they had managed to put so many different mechanics into both games, and still have everything feel so smooth.

It's really impressive just how much they shoved into those games, but I do wish they had spent a little more time/money on writing an interesting story and developing the characters in ways that don't feel like the most cliche way possible. Like I said though, we're there for the gameplay, and it definitely has the gameplay.

I'm currently playing through it with a gamer buddy. The gameplay isn't challenging, but it's definitely fun. I was talking to someone else who tried playing with their non-gamer wife, but they had to stop because she was getting frustrated with things.

I agree about the writing and it being the weakest point. Then there's the part with the elephant:

The level design and environments are excellent, though. Between these and the gameplay, the game is still a joy to play through, even if the writing sours the experience a bit. Split Fiction is definitely on my watchlist, and I plan on picking it up during a sale.

I think the reason it is so well received is due to the fact it connected a wide range of genres in a single game, connected them in a coherent way, and executed them all very well.

Most games have trouble just trying to do their genre well

You're right. We have played many borderlands to completion and have played PoE2, Warhammer Vermintide, Darktide, and other games with my wife. But those she wants to go back to, or are memorable to our shared time playing them, are the more casual ones that offer a light-hearted experience or are simply more inviting: Nine Parchments, Human fall flat, Untitled goose game, Trine series, Bombsquad, Moving Out, Operation Tango, etc. This includes A way Out, It Takes Two, Split Fiction. But we also take singleplayer games together, like Monument Valley, Gris, or Chants of Senaar.

I believe something that gets ignored often is the theme of games having a big influence. A lot of co-ops that seem to have decent gameplay is about "dark evil world where everything is fucked up and the main loop is about multiple approaches to violence".

So as you said, accessibility gives it a larger public and its creative experience makes it memorable enough that couples recommend it to other couples and so on.