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.