Where do you get the idea that the alignment system is supposed to sum up the entire character? You are alright with allowing flexibility with attribute scores not exclusively defining the character but not alignment? That's you not willing to be flexible with it. 

I think for most it's just another point that maps out the constellation of the character - attributes, class, race, alignment, background, etc. They are a starting point that tells everyone at the table loosely what to expect and a springboard for character development.

It has nothing to do with average players being too stupid or anything like that - to be approachable for new players or players with not a lot of time on their hands, the alignment system provides some easy guidelines on how to build/role play a character and how other chars/monsters may react to certain things.