Have you played Pandemic as per the OP?! Well Pandemic once you beat it has 0% replayability. There is no story, there are no multiple villains, there are no heroes, etc. It's just a bunch of cards that dictate which cities will become infested with one or more of the 4 viruses.
For certain CO-OP games, perhaps this is true. But specifically the example in the OP with regards to "solving the puzzle" was mentioned for Pandemic. You assume that I was talking about ALL CO-OPs... When in my case, I was specifically talking about one game in particular (Pandemic).
Remember Pandemic is a EURO CO-OP. No story, no characters other than six or seven role cards which are usually needed to beat the game. Without one of those roles, the game is very challenging to BEAT. But with five (5) out of six (or seven)... The game can be beaten reasonably EARLY...
And that's the thing... IF you are going to WIN this game... It has to be EARLY before all the viruses start infecting too many cities and the level of contagion is too high...
And IMHO that is boring: win early (in a play-thru) or lose.