That’s because “outsider” wasn’t used as a Game term in Tolkien. He was a demigod created at the beginning of time, who lived millennia in what is middle earth heaven. He is clearly, obviously nonhuman, nor a member of any of the other races the gods made. Tolkien suggests that there are other casters in the world, other than the 5 “wizards” all of whom are outsiders. They are called sorcerers IIRC. That does not at all suggest that their abilities map to Wizard or Sorcerer in 3.5. There are really quite a few “caster types” in ME. From the elves, who clearly use magic but it is very rarely seen on screen, to Finrod Felagund who enters a magic chanting battle with Sauron, to the sorcerers Gandalf refers to, to the dwarves of yore who made “mighty spells”. None of them look like any kind of Tier 1 caster.