RPGMemes

Its cause you really only need one person good at a skill in the party. Once you have one person with high thievery (or, any other skill, really), each addition of another character with that skill is worth less and less.

While, combat focused classes are kind of the opposite. Hard to have too many combat classes in most dnd-likes, and the more classes you have narrowly focused on combat, the better the party is at that task.