Why then do mobs not tunnel vision the healer? That would make more sense, since without heals the group cannot succeed. It's a game and to solve the "I'm gonna hit the guy who hits me hardest" issue they implemented aggro / threat.
If I was a mob, the guy in glistening plate with a sword and shield who is bashing my face in every so often with his shield, while making me weaker and not giving any quarter for me to pass them by is who I focus on.
The ferocious bear who's ripping me apart with it's claws and growling protectively over their friends, gets my attention.
The undead knight who's seen much war in his days, who attacks with necrotic energy and steals my own life energy to supplement his own is who I'm going after.
The bastion of holy radiance, protecting his allies with holy energy and smiting me with blessed shield and using holy energy to fuel his wrath will be my target.
The drunken brawler nimbly avoiding my every attack and shrugging off the ones that manage to land will be sure to feel my wrath.
See, it can make sense. In reality, you wouldn't always attack the one who's hitting the hardest. You may choose to go after the leader - the one standing on the front line leading the charge, taunting your from afar just to get you angry. You would assess the situation and logically determine who the best target is. Because this is a game though, they have to add mechanics to make those kind of ideas work. And again I ask, what other type of high fantasy game has the stereotypical protector doing more damage than the assassin or the berserker?