Have you ever played a cooperative game with your friends and thought that maybe you would enjoy it more, if you would play it alone? If so, it might have been because you did not play a cooperative game, but a game with cooperative mode. The difference is that while the former is designed with player cooperation in mind, the latter has it as an afterthought.
I first really realized that when playing Dying Light with friends, where we were getting constantly scattered, doing our own thing, while closing in on the next mission objective. The reason why is that we were doing fine on our own and while having more people helps with clearing groups of zombies, it does not make that much of a difference. The fact that every player gets their own loot did not help to encourage cooperation either.
Factorio provided a similar experience. Everyone had their own plan in their head and so built their own thing and somehow connected it together. It is the kind of game where unless players are all on the same page, they will get more in each others way than help each other. Both these games I enjoyed more when playing alone.
Things are different when you play a well designed cooperative game. In games like Left 4 Dead 2 players stay close to each other, because they have to rely on each other. When a special zombie gets you, you can’t escape from it by yourself. Even regular zombies can be a problem in large numbers, so every player guards their hole in a wall to not get overwhelmed. In PayDay 2 someone has to keep an eye on hostages, while someone else answers the phone, while someone else goes fix the damn drill. In Foxhole you rely on other people with everything, from pushing the front to getting a rifle to shoot with. In these games you play as a team, instead of playing alone in a close proximity to each other.
It is true that every game is fun with friends, but I think some are more fun without them.