So wizards are common and don't rely on magical items in D&D? Ironically enough, the characters I am referring to actually own only a handful of magical items, with the possible exception of Turjan.
I'm less familiar with Dunsany, but Vance, Leiber, and Moorcock all cast spellcasters in protagonist roles. Turjan, from Vance, knows over a hundred spells. Leiber's Grey Mouser is a magic-user/thief; I don't know how many spells he knows, exactly. The Lankhmar stories in which he appears virtually defined the D&D urban environment. Moorcock had several fighter/magic-user protagonists, including Elric and Corum. One of Elric's adversaries was a powerful sorcerer who used no magical items, and could cast a startling variety of spells by bending reality, at the snap of a finger. I'm not sure what you're using for reference, but those stories are clearly some of the most infulential stories which inspired D&D, and have been stated as such by Gary Gygax.
Turjan knew four spells. Certainly he had a wealth of them in his library, but he could memorize four of them. The vast majority of Vance's books were on swashbuckling and magic items and people who had just enough power to get themselves into enough trouble that they couldn't get out of.
Grey Mouser was a thief who also knew a handful of spells. The emphasis was never on the spellcasting.
Elric never cast fireball. He could do a grand and powerful ritual that was hilariously draining to do magic, certainly! But he was never anywhere close to a D&D wizard. Oh sure, his antagonists could cast plenty of spells with no magic items, but they weren't PCs, were they?
I never read Lankhmar, so you have me there!
Maybe you know better than Gary what D&D is.
You were doing better when you were using the tragic death of a NASCAR racer as an example.