As a counterpoint, while yes sword + shield is more efficient than two swords in most cases, the list of cases where the two swords could prove to be more useful and the list of situations a party of D&D adventurers might get into overlap quite well. For one, there's the clockwise stairs problem in castles, which would make being ambidextrous an advantage.

Second, the name of the game is Dungeons and Dragons, and their definition of "Dungeons" is pretty much "old, decrepit, hazardous, and underground". A large shield is going to be your worst enemy when trying to clamber around obstacles in such an environment.

Lastly, yes, anyone can wield two weapons just by picking up two blades- at a hideous penalty. It takes a substantial investment of training (in the form of feats) to be any good at it. A 2nd-level Wizard can spray fire from his hands, strike down distant foes with homing darts of force, and give a man the size and strength of an ogre. A 2nd-level Cleric can call upon the blessings of his deity to heal wounds, withstand the blazing heat of a desert, and understand any spoken or written word with perfect clarity. A 2nd-level Fighter . . . can hone his skills to where he can swing two swords at the same time without them getting in each other's way too much, and even use the off-hand weapon to parry attacks.

"This is probably a stupid question, but . . ."