Why Earth's mightiest heroes have never mentioned Coulson again is anyone's guess. But there are certainly some behind the scenes reasons that could have played a part. The film and TV divisions of Marvel are now separate, and not exactly working as one cohesive unit with a unified vision. We have heard in the past about filmmakers having issues with the Isaac Perlmutter leadership that is now in charge of the TV side of things, so it is unlikely that the two divisions would work together to achieve something as small as having the Avengers see Coulson again. There is also the matter of Agents of S.H.I.E.L.D. never really being a hit or must see television for fans of the Marvel universe. That's unfortunate, because the show has improved in quality year over year and become a good show that plays with fun comic-booky concepts.