Because you can shuffle people into little categories. Humans like to categorize things.

Never mind that personality psychologists deride it.

In fact, the newest model, HEXACO, goes off the Big 5 model, which is a spectrum model instead of a binary model. Even HEXACO is beginning to fall out of favor. The six dimensions are Honesty/Humility (a measure of how prone you are to be deceptive, sociopathic, and unrealistically self-aggrandizing or self-injuring), Emotionality (a measure of anxiety, neuroticism, sentimentality, emotionality, moodiness, and what might be best visualized as 'how much you are like a little chihuahua who is shaking, running around, and pissing itself'), eXtraversion (this is pretty straightforward), Agreeableness (how much of both a 'team player' and a vaguely over-dependent ninny you may be - if you're super-agreeable, you are probably a doormat, and if you're not very agreeable, you are probably an asshole), Conscientiousness (do you rack disciprine?), and Openness (are you liberal, well-read, educated, worldly, appreciate the intellectual and the aesthetic? You score high on openness. Are you a conservative hide-bound ignorant redneck? You score low on openness.)

Never mind, of course, that the real answer is that even if you could separate humans into broad personality traits, those aren't even necessarily completely inherent to the person - I mean, sure, there are tendencies, but culture and our own reasoning and deliberate action has more of an effect on it than we think.