Considering some 'games' can be very good, but lack advertising or support from their publishers, you may want to alter your perception. The people who develop the game and those who publish the game are two different groups and it takes support from both sides to make a game successful. Claiming games deserve whatever sales they get, if you're implying that games should sell based on 'how good they are', that is a very ignorant view.
Oh no, its not like that at all. Its more like games are fairly risky purchases and therefore the qualities about the game and how the game is recieved determine how well it sells. Some qualities include word of mouth, advertising etc which tell people about the quality of a title and others like peer pressure and being able to play with all of your friends in online multiplayer because everyone is getting the title is another. You only get out what you put in so pretty much everything gets what it deserves. If 5M people think that the name Halo deserves purchase on day one this year its only because of the overall good qualities of the Halo series that makes it happen.
Heres a list of qualities which go into sales of a title:
- How fun it is to play
- Value for money of the experience
- Word of mouth
If a game ticks all the above boxes then it'll get good sales. I don't think its important to weigh on how a game is fun so much as to ask whether or not it is fun to play.