Alright, here's the thing.
There's basically three levels of comic book crossover, right?
There's your basic "Ambient" crossover, which happens in the books all the time. Matt Murdoch happens to be defending an X-Man in court, Peter Parker covering the Olympics in a Luke Cage book, stuff like that. That's what most of Phase 1 was based on.
Then there's your basic team-up book. Avengers, Teen Titans, Justice League of America, stuff like that. Threats that require a larger, concentrated effort, but it's still mostly relegated to the individual book, and isolated from the universe at large. That's what Avengers 1 and 2 were. Awesome in their own way, but there's a good chance we're never seeing the Chitauri or Ultron ever again.
Then you have your huge, ultimate EVENT crossovers, which can be lead up to for years, span several books, have massive and lasting ramifications for the universe at large, and can change the way the books themselves are percieved and even sold. DC's crises, Flashpoint, the 2006 Civil War, stuff like that.
That's what Infinity War 1 and 2 are being built up to being.
The MCU as a whole is built up, not just of the individual characters' stories, but the over-arching meta-narrative of "Can these movie makers even pull it off?"
Nobody REALLY cared about Loki and the Chitauri in Avengers 1, it was all about whether they could pull off a crazy stunt like "Can we make a movie that's a sequel to five other movies at the same time?", and everything after is about escalating that insane game of Producer's HORSE.
The problem is, there's really nothing left to do. No real narrative stone left unturned. Anything the MCU does after Infinity War can only be a lateral move, so there's basically no reason left to care.