There are games that are just as great as MGS, but only few people have heard about them. You can't compare these to piles of mud, people just don't know about them. That's why they don't buy them.
I'd rather have a game that few people own. That's why that's what rare means to me. Maybe your opinion is different. I agree that MGS: TTS is in some sense hard to get. But it's not the game I would buy for a collection.