Why? Because everyone knows sorcerers and wizards blew up the damn world. (Well they had help sure but hey they were the ones doing the blasting!)
How? Divine War. It tends to make a lot wizards and sorcerers feel powerful when you can summon armies, cast spells that slay with a word, or just call on eldritch powers to turn enemies to stone, werewolves, etc. Not to mention many women become Hags because they used sorcerous power.
So while some might be willing to help aid others and do go works, they still believe they are the superior force in the world since they control arcane magic. And while Divine Magic is cool and powerful, arcane magic makes the world go round. (People need battle mages, tribes need "tribal" shamans, etc.)