We at Entertainment Stew have really gotten into Tabletop gaming(check out our homebrew tabletop adventure The World of Vala). I enjoy the basic structure immensely, the very detailed and vast shared world that you can just exist in.
Within these worlds, players are able to experience all sorts of smaller stories and adventures in that universe. I’ve always been a bit disappointed that such a thing never took off in video games. Video games have never achieved what I’d like to call “horizontal adventuring,” with many similar games exploring different stories in the same setting.
In most cases, video games have “linear adventuring” i.e. sequels, but the subsequent games vary much in technology and game mechanics that you can’t get that feeling of “many adventures in the same universe.
Of course, what I’m referring to here are often called "campaign settings" in tabletop RPGs. Some examples (from D&D) are Blackmoor, Greyhawk, Dragonlance, Dark Sun, Ravenloft, etc. The main purpose a campaign setting is to provide a backdrop tying the different adventures/modules of a campaign together, giving players a place to sell their loot, buy new items, and train between adventures.
Since video games rarely (if ever) allow you to reuse and continue to improve a player character from another game, there really isn't much need for something like I’m describing. There is no downtime between games for you to explore the larger campaign setting before setting out on the next adventure. Since you generally can't reuse characters, there's really no point.
I think, however, that I actually maybe asking a slightly different question, more like: Why isn't anybody making video games where you can reuse your character as you play different adventures, getting more and more powerful as you go, and also doing things in between (and also reusing game assets)?
There are some token gestures regarding minimal character reuse, like Mass Effect, but that's not really the same thing. All of D&D is built around the idea of stringing together miscellaneous adventures using the same (or very similar) rules, and slowly but steadily leveling up your characters under the supervision of your DM. For tabletop, a DM can adjust things as you go. For video games, mixing and matching adventures within a larger campaign is a game balance nightmare, and there are lots of additional mechanics and tools required to pull off even a bare-bones platform of that sort.
The closest the game industry has come to that is probably Neverwinter Nights (2002), which was less of a game and more of a build-your-own-adventure platform. You could roll up a party of characters, play through the base game or a community-made adventure, and then reuse the same characters -- keeping all of your equipment and experience -- in a higher level expansion or one of the hundreds of community made adventures. In fact, the NWN modding community actually made several modules with shops that could be used to resupply between adventures. You could also have a live human DM monitor your progress and adjust things if needed as you played.
NWN as a platform was a mixed bag. Unfortunately, the vast majority of community content was pretty low in quality. Thankfully, due to the huge volume of community content, there are more high-quality adventures available than most folks would ever be able to play. If you pick your content carefully, an individual or a small group could indeed roll up some new level 1 characters and then proceed through a series of (possibly unrelated) modules, where your character kept leveling up and acquiring 100% transferable gear throughout the entire campaign. And then you can go back and roll up some new characters and go on new adventures over and over again.
Why don't more game devs do this? Unfortunately, the cost and hassle necessary to do all this are astronomical compared to the revenue generated. It's always possible some indie will go down this road. But considering that RPGs are already one of the most mechanics-heavy (i.e labor intensive) genres, adding all those whistles and bells on top of a simple, functional one-time experience is generally impractical. For NWN specifically, that resulted in a rather bland default single player campaign, important tools released years later, and many other important tools left to the community to provide. There were lots of gaps, despite the fact that they had an army of people working on that game. That said, the platform itself works great for its intended purpose, and the abundance of community content makes it a great option for playing something similar to a D&D campaign to this day.
(Photos courtesy of Creative Commons)