There are probably several reasons for why many might feel this way. Some could be very simple explanations others could be rather complex. Long post incoming.
Perhaps the simplest explanation is that players are hyping themselves up to levels to expectations that are simple unreachable. Once they finally play the game and it fails to meet to up to such lofty expectations, they may feel disappointed that the game didn't quite satisfy them, even if the game is actually pretty good.
Another, more complex answer might be due to the amount of content, or the quality of content provided in the game. These days, with ballooning budgets and heavy focus on art and graphics, the amount of content provided by a game is shrinking, despite the fact that the number of hours in a game may be increasing. Or the content inside the game itself is lacking in quality or failing to appeal to players. Could one say the extra side quests available in the original Dragon Age from the various wanted posters/chests in towns has the same sort of depth or complexity found in the side quests of those in Baldur's Gate? I wouldn't say so. Due the fact that developments costs have risen, and the time it takes to develop the main game, many developers opt to create additional content in often quickest way possible with very little afforded development given to them. This is why we often get games with extra content with little more than showing off the battle engine that took several months to polish and balance. Or simple quests fetch and kill quests that are utterly devoid of depth or strategy (see AC3's Assassin Contracts as compared to those from AC2 or ACBro). And even more often, these sorts of extra content that were developed are given in extreme amounts and usually are the only content available (The huge amount of Cieth/Boss kill quests in FF13). This sort of content is appealing to many players, but the sheer number of them, and little else is really the result of a lack of development time to create a good variety of content such that everyone can find something else to do. Instead, these days many developers turn to trophies and gamer scores as a method of adding content, when it really just s shallow attempt to extend the play time of the game. Or they add multi-player to help curb the rate at which games are traded in.
The Dead or Alive series of games have often been seen as games lacking in content. Dead or Alive 2 had all its costumes unlocked at the start while Dead or Alive 2 Hardcore added a few extra costumes and locked away all the old ones. Yet Dead or Alive 3 and Dead or Alive 4 release the lack of content between them and its prior games shows. Dead or Alive 5 is perhaps the most content-rich Dead or Alive game yet. With half a dozen difficulties for each of three main modes, plus tag teams, a more robust story mode, hidden characters to unlock and 500 titles to collect. Yet its not really all that fun to unlock all those titles, and not every player is going to want to tackle the hardest difficulties of the game, or even attempt to play all the modes. With the Soul Calibur series, more recent games like SC4 and SC5 are lacking in content to the older iterations, despite new modes like a character creator with more than a hundred different pieces of equipment to find and/or collect.
And even if said games had access to a large amount, and a large variety of content, its not likely every player will be able to access them. Many games (in particular the more recent Final Fantasies) often have huge barriers of entry before one is able to tackle that content. One example is Final Fantasy 10's and Final Fantasy 12's extra bosses. These guys are incredibly difficult to beat, and even getting up to them can be considered a chore. For some players, its not very fun, but for others it can be appealing. Final Fantasy 13-2's extra content (and even main quest) have a few barriers of entry that make it difficult to access, a there's quite a bit that may not be all that appealing either way.
Finally, there's also the possibility that in the end, the game just doesn't satisfy you. You play it, you take on some of the extra content, you beat the game, and yet... you don't feel very satisfied about it. It could be because the game itself is fairly mediocre, or it has a terrible ending. Or simply, you're using it much in the same way as food; feeding a habit, but not a craving. There's a possibility that you'll go through several games in the same way, some you may just not beat because you end up too bored with it that you can't be bothered to finish it, even after putting dozens of hours into it. You can't think back upon the games you've played over the past few months (or years) except for perhaps a few. And those that you've felt truly satisfying are the ones you can remember. Some of it may be nostalgia, but if you try thinking back to more recent memories, it probably isn't some that easy to explain. In essence, you're yearning for an experience that you really enjoyed from a past game, and hope to feel that same kind of experience again in a future game. But nothing you've yet played, has given you that same sort of feeling. This isn't the same sort of feeling like an addiction, mind you. Rather, its more akin to yearning for a good book or scratching an itch that finally goes away. Its the same sort of feeling that other people in other hobbies have when they are unable to satisfy that craving. Like a fisher who once caught a large fish and seeks to do it again, or aim for something bigger, but has yet to grab it. That's the sort of feeling you or other players may be experiencing, and they have yet to find an answer for it.
And it could be none of the above. Or it could be all of them. Still, its important to look at the games objectively and critically. What you think about them should matter first and foremost, before what others think about them. Some people tend to get caught up in both the hype and the negativity that might surround a particular game and it may seize upon your thoughts and cluster yourself into a hive mind mentality. Peer pressure is pretty powerful, and even affect your opinion on things that you might find enjoyable. Best to block out others opinions and rely on your own until you complete the game, before you rely on others to tell you what to think. Just make sure you think critically about the game before you go buying it. You don't wanna waste 60 bucks on a game you might not enjoy. Even if you can trade it in.
