There used to be one image. One. You'd see it on a bus stop, at the cinema, on a friend's bedroom wall, and it was the movie — Jaws' shark rising toward that swimmer, E.T.'s glowing finger, the Star Wars cast arranged like a painted myth. You didn't need a title to know what you were looking at. That's basically extinct now, and it happened slower than most people think, in stages, over about three decades.
The first crack was technical, not creative. Through the 90s, the illustrators who used to paint those posters by hand — Drew Struzan foremost among them — got quietly priced out by Photoshop composites: cut out the actor's head, drop them on a moody background, done in an afternoon instead of six weeks. Cheaper and faster naturally means more of them.
The multi-poster format itself is older than people assume, too. Ocean's Eleven gave every cast member their own individual portrait back in 2001, X-Men did the same the year before that, and international/regional variants have existed since classic Hollywood distribution — this isn't a 2010s invention. What Marvel actually did, starting with the Avengers era, was industrialize it: turn "the poster" into a content calendar. Teaser poster, a portrait for every hero, an IMAX exclusive, a regional cut for three markets, a "final" poster that's really just the fifth one that month. Every other studio saw the engagement numbers and fell in line.
So there's no single villain — it's illustrators losing work to software, an existing ensemble-poster format getting industrialized for social media, and a marketing calendar that needs something to post every week.
Struzan himself gave the whole thing a proper ending, rather than just fading out. After largely stepping back from studio work, he came back only for friends who insisted: a special poster for Star Wars: The Force Awakens in 2015 at J.J. Abrams' request, and Guillermo del Toro's Hellboy films, made because del Toro reveres him. Then he retired for good. It wasn't a studio deciding illustration was over — it was the last great illustrator deciding, on his own terms, that it was.
Full disclosure: the person writing this has Struzan's name tattooed on their arm. Some things you don't get over.