Here's my theory on why Charisma is a class in D&D games:

 

This game was created by, and for the most part played by, geeks. Such people may not think of themselves in this way, but since I played the games for awhile, and knew that I was a geek, it's pretty much assumed others were as well.

 

Being a geek tends to indicate that while that person is smart and creative, he/she is woefully lacking in self-confidence and most social skills. Probably why such people played D&D was because here was a chance where a person could be another character, another person, one that had oodles of confidence, attractiveness and strength, something geeks lack in real life.

 

So one thing geeks like to do is quantify things, and D&D is all about that. Every facet of each character can be broken down into simple numbers. The higher the number, the better the character is at any one attribute. If that's the case, then why not quantify how charming, attractive and encouraging that character can be?

 

That's why Charisma is part of the D&D world. Geeks want more of it, and here's there chance to get some in a fantasy world with a simple roll of a 20-sided die. Otherwise, there's no need for Charisma as a statistic. Besides, it can be assumed that D&D characters geeks create would already have enough confidence and charm. Charisma is really an innate ability, if we go down that path.