
He did so in Doctor Strange and has carried on doing so since. However, to answer why Wong is acting like he is in the Spider-Man: No Way Home, Wong is a mentor to Stephen. For such a big event it would have at least a mention so we can assume it hasn't happened yet. Then in all of the media Strange appears in afterwards: Thor: Ragnarok, Avengers: Infinity War and Avengers: Endgame no mention is made of Strange having become the Sorcerer Supreme. Earth has no Sorcerer Supreme to defend it. Word of the Ancient One’s death will spread through the Multiverse. Wong: You might have a gift for the mystic arts, but you still have much to learn. It is later spelled out for us at the end of the film that the Sorcerer Supreme "job" is still vacant with interviews pending.

With its Master gone, it needs another, Master Strange. The Ancient One: You defended the New York Sanctum from attack. At the end of Doctor Strange he has been made Master of the New York sanctum. Strange is yet to become the Sorcerer Supreme in the MCU.
