I get why they aren't in the individual games though, because then people would spend their days constantly playing them to earn themselves keys. I get that it's a pain to find people, and I am usually on that same side of the luck too, that whenever I want to play nobody is around to play either HM or POTC.
I have no idea how coding and everything works, so I don't even know what would be feasible, but maybe the individual games could have some sort of trading system, sort of like an arcade where you play the games and earn tickets and your tickets are redeemable for prizes, except in this case your "tickets" can be redeemable for keys. Again, just a thought as I don't necessarily agree with putting the keys on the individual games, but I do see the frustration of not being able to play multiplayer games!