Ever since Marvel started making television shows, fans have been wondering when and if there was going to be a crossover between the movies and those television shows. It turns out that isn't going to happen anytime soon, if comments from Avengers: Infinity War and Avengers 4 co-director Joe Russo are anything to go by. Première asked Russo specifically about The Defenders making an appearance during a set visit, and he said the following (pardon any rough translation errors).
"We have considered everything. But the trick is that we already have at least 60 characters with whom we must tell a story! And it's hard enough to communicate with Taika Waititi, Ryan Coogler, Scott Derrickson, Peyton Reed, and James Gunn at the same time, not to add all the showrunners and TV crews to this. The latter is also led by totally different people. So it's practically impossible. Our job is to focus on the Marvel film world and offer a satisfying climax."
The short version is they couldn't come up with a way to make it work. They already felt like they had so many people they had to consult — the various previous and future directors — to make sure all of the characters translated properly. To try to throw the television characters in there would be too much. At this point Marvel TV fans would probably take an acknowledgement and be happy.
Summary: As the Avengers and their allies have continued to protect the world from threats too large for any one hero to handle, a new danger has emerged from the cosmic shadows: Thanos. A despot of intergalactic infamy, his goal is to collect all six Infinity Stones, artifacts of unimaginable power, and use them to inflict his twisted will on all of reality. Everything the Avengers have fought for has led up to this moment – the fate of Earth and existence itself has never been more uncertain.
Avengers: Infinity War, directed by Anthony and Joe Russo, stars Robert Downey Jr., Josh Brolin, Mark Ruffalo, Tom Hiddleston, Chris Evans, Chris Hemsworth, Jeremy Renner, Chris Pratt, Elizabeth Olsen, Sebastian Stan, Benedict Cumberbatch, Paul Bettany, Chadwick Boseman, Benedict Wong, Zoe Saldana, Karen Gillan, Vin Diesel, Dave Bautista, Pom Klementieff, Scarlett Johansson, Don Cheadle, Tom Holland, Bradley Cooper, Sean Gunn, and Anthony Mackie. It will be released on April 27th.