I can put on my game designer hat and try to answer some of this. Bear in mind that my game designer hat is new and relatively small.
Offhand, there's one game that I can easily think of that tried to actually simulate a tabletop game, and that's Neverwinter Nights. In NWN, a team of players running with a live DM could easily deviate from the DM's planned adventure. About the only thing you can't do on the fly is make new maps, but a DM who makes a "placeholder map" and jumps the players there and tells them to "imagine the setting" can still plunk down orcs, treasure chests, and whatever else needs to be plunked down. Players who want to deviate from a dialogue tree can ask the DM to play a creature live, and the DM can do it.
It won't be as polished as the adventure that the DM had planned, of course, but that's not all that different from a tabletop game in which the players opt to go someplace completely different -- the monsters tend to be right out of the book instead of crafted specially, the NPCs don't have as much interesting dialogue, and everything takes a bit more time because the DM is coming up with it all on the fly.
As for other games, they give you a character to play with a varying degree of story and freedom... just like a tabletop game.
- In the current tabletop game I'm playing, I was informed that I had to be human, that only one person in the party could be a magic-user, and that our backgrounds were to a large degree determined for us -- we got to choose from options presented to us.
- In a game I played in a few years ago, the DM, running a module, railroaded the players mercilessly through the module, even to the point of saying, "See this? (raised hand) This is the hand of plot. The hand of plot says that you can't get inside this building until you go clear that other building."
I suspect that some people will now try to cast aspersions on CRPGs by saying, "That frustration you experienced was because those DMs DM'd as though they were the computer in a CRPG." No. They DM'd like DMs. That's my point.
A tabletop RPG has, in theory, more freedom than a CRPG. A tabletop RPG also has more potential areas for things to go off the tracks -- the computer that people attack for its inflexibility never forgets how Bull Rush works or gets into an hour-long argument about whether walking along the very narrow ledge is a Balance check or a Climb check.
But in practice, as in most things, CRPGs aren't as far from tabletop RPGs as most people would think. They aren't identical -- it's almost always easier to have a fight against 20 unique bad guys in a CRPG than in a tabletop game, and it's almost always easier to roleplay a cocktail party in a tabletop game than in a CRPG -- but they are very very similar. In both cases, the person running or designing the game makes some choices that balance exploration freedom versus the story that the designer wants to tell. In a CRPG like Oblivion, the designers wanted the player to have the freedom to create his or her own story. In a CRPG like Star Wars: Knights of the Old Republic, the player has a lot less freedom, but the story is a lot more focused (and much stronger) than in Oblivion. Games like Baldur's Gate 2 strike a middle ground by giving the player a long-term goal (story) but a lot of freedom about how to get there (exploration).
In the tabletop games I played in, the DMs made choices -- the first DM wanted a low-magic, human-centric game, which is why we all started out as peasant kids. The second DM wanted to run a game right out of the module without doing any work of his own. In theory, the DMs could have decided to change what was over the hill because of what the party wanted, but in practice, it didn't happen. People generally make choices and stick with them.
The difference is that in a CRPG, the choices all have to be made beforehand -- how much dialogue to give a given character, whether to make someone romanceable, how many options to give the players. You play the odds -- most players will take this obvious path, some will take the less obvious path, and one or two would think to try something crazy. You design the obvious and less obvious paths, and you look at the crazy path and figure out whether you've got the additional resources to make it happen. A lot of times, you don't, and that's life -- just like a DM deciding on the fly that he's not letting the players get into the warehouse unless they talk to the guard, because he doesn't want to have to come up with some other way for the players to learn the information that the guard gives them.