This is near and dear to me, and I'd say it goes beyond just co-op. We used to get "the whole package" with a game. Arguably Call of Duty is one of the few still offering it. We used to have games with campaigns and multiplayer. Story mode and challenge rooms. Other modes of play sitting alongside the main event to round out the package. Now developers look at any data point to see how many people are using it, and if the number isn't high enough, they cut it. But that's a mistake. Most people might only dip their toes into these side features, but they can usually be implemented relatively cheaply (because of asset reuse), and they can add a ton of value even if most players don't spend a lot of time in them. Co-op is one of those things.

The games that used to offer these co-op modes tended to stop getting attention from their publishers. Then once they've got a multiplayer mode, they try to make it a live service and monetize it instead of just letting it be. I was screaming at my monitor when I read that Naughty Dog open letter about canceling the Last of Us multiplayer game that said they had two choices and neither of them was making a multiplayer game that they just sold for a box price and didn't manage as a live service; the possibility, seemingly, had never even crossed their minds. Co-op games can't just be a campaign you play through once with a friend; they have to be PVE grinds where you play the same content over and over until the next pack of it comes out in a few weeks. The likes of a Baldur's Gate 3 or an It Takes Two feel rare by comparison.