Honestly it's always baffled me why there aren't more PnP-based RPGs out there. You save a lot of effort on creating lore and balancing game rules, plus you have an already established fanbase, however small. And tbh, most game devs aren't very good at writing lore or world-building, so using PnP settings would definitely be an improvement in most RPGs imo.
It isn't that simple.
In terms of game rules, some rules that work in tabletop RPGs (TRPGs for short) do not work so well in computer RPGs (CRPGs for short). For example, you have stats like Charisma, which mainly exist for role-playing purposes, while having little mechanical effect; that does not translate well to CRPGs. (Granted, D&D 3.x did add uses for the stat, but its roles could have been given to another stat and the number of stats reduced from 6 to 5, without losing anything from a pure combat perspective.) If you have Charisma affect NPC interactions, it is too easy for the stat to be either useless or game breaking (from what I hear on another sub-forum, the latter can happen in Morrowind).
Also, the d20 + attack bonus - AC to hit works reasonably well in a tabletop game, particularly if the level numbers are kept small (though it can still be frustrating if you keep missing); when it comes to CRPGs, however, there are better options available. For instance, divide the attacker's accuracy by the defender's evasion, and the result is the chance of hitting; that formula handles larger differences much better than the d20 system approach. (I note that the AD&D/BG THAC0/AC approach is isomorphic to the d20 approach; it's just some numbers and signs that are different.)
I could also point out that the whole d20 mechanic falls apart more and more as you get to higher levels. For instance, at level 4000, a 1% difference in level means a 20 point difference in attack bonus, which in turn can mean the difference between "natural 2 is a hit" and "natural 19 is a miss", making it impossible to balance the game without those stats becoming meaningless. You can actually see this in Throne of Bhaal, where enemy THAC0 is so good that AC becomes useless (unless you manage to improve it far beyond what you would normally encounter). Compare that to Disgaea, where a small difference in level or stats is not a big deal, even around level 4000 or so. (The game still has its balance issues, but can you name a TRPG that can handle numbers that big while remaining at least as balanced as Disgaea 1?)
Also, having your stats set in stone for the whole game is, IMO, not fun; I like to see my stats improve, and I don't like the initial random stat role to set my character's destiny for good.
Edit: I also wish developers would stick to turn based combat instead of copying the Infinity Engine's system which combines the worst aspects of turn-based and real-time combat.