Multiplayer isn't the problem, it's a symptom. The problem is that game companies are obsessed with anti-piracy. The most proven method for preventing piracy is force the player to log in their central server every time they load the game. Players tend to resent this sort of thing (Diablo 3, anyone?) unless there is something tangible to be gained from it. Then the marketing guys come over and say "Well heck, let's just add multiplayer mode so that they'll all be happy to log into our central server!" And in the boardroom there was much rejoicing.
But then they noticed an unsettling trend: The pirates simply didn't give a crap about multiplayer and simply ignored the whole central server thing that was so essential in stopping piracy. They shrugged at the impotent multiplayer content that no one was going miss anyway and just played the game entirely offline. Soon afterward the developers decided to just take their ball and go home. Instead of multiplayer being something tacked on to the side it was now the core of the game. Ha ha, take that, pirates! Now there is nothing left to our game but a multiplayer mode that you can't access because you're a filthy pirate!
Nevermind that they were chasing off paying customers faster than the freeloading pirates. A pyrrhic victory was still a victory in their eyes. And so it became that the banner of anti-piracy became the depressing path of making games with flimsy cores that were held up by nothing more than cheap multiplayer spackle to force server log-ins, because having that strong anti-piracy system is far more important than having any actual game content worth paying for.