GURPS (and other systems) use 3d6 for all skills and combat. It gives a more reliable result and means that a person with a reasonable amount of training and experience can be relied upon to hit a target more often than not. Combat becomes less random.
D&D is probably stuck with the d20, though, especially as the whole 'd20 system' thing seems very popular. It means that the die you roll has a more random spread than rolling several dice together. This is just something you have to live with unless you want to design a completely different system.
