Headbutting in soccer

Remember Zinedine Zidane headbutting Marco Materazzi in the 2006 FIFA World Cup final? Of course you do. Headbutting is actually quite widespread in the game (for anecdotal evidence, see here, here and here). I have trouble understanding why that is though. Why don't they punch instead? The penalty for headbutting someone is the same as for punching them: immediate expulsion from the game. If someone made you mad enough to headbutt them, they probably made you mad enough to punch them, and if you find yourself in a fight, your first instinct is to punch not headbutt anyway.