Realistically, it is used so much for the same reason projectile weapons are used in the real world -- it is very, very effective at what it does. It weakens the enemy's saving throws, weakens his attacks so it improves your saving throws against them, makes the enemy incapable of using his or her most dangerous spells, and can kill outright. Why wouldn't someone use it if they had access to it?
If a villain had time and resources to prepare for the fight and did NOT equip themselves with the best items and buffs, wouldn't we say they'd been handed the idiot ball?
Respectfully,
Brian P.