See, my reasoning is two-fold.
One, most people don't stay for the very end of the credits, but usually stay for the mid-credits scene. From a narrative perspective, why not finish the story they were just watching? This is my biggest beef, honestly because from a narrative standpoint on JUST the movie on its own, the end-crdledits scene males more sense to put first.
Second, most of the teasers for next movies, while fun, are usually fan-service for nerds like us that recognize Thanos or the Infinity Gems or future Avengers. Judging by the movies so far, the teasers were not important to the narrative of that teased movie. If it was, it was explained in that movie.