It’s true that Marvel Cinematic Universe fans have gotten a taste of magic in the past, namely with the introduction of Elizabeth Olsen’s Scarlet Witch, but Doctor Strange is the first film in the canon to dive head first into concepts of the multiverse and the idea of pulling energy from other dimensions.
While Jon Spaihts didn’t directly mention it, there is another important argument to be made for why Doctor Strange really did need to be an origin story: the introduction of magic.
Understanding this part of the sci-fi/fantasy world is integral into understanding Doctor Strange as a character, which is why it makes sense to package their introductions together in an origin story.