It's too damn big.
I mean, no doy, right? All but one of Marvel Studios' previous "event movies" have felt at times overstuffed, and they were only trying to juggle a fraction of the characters - Avengers: Infinity War is attempting to balance an unprecedented twenty-two, and that's not even including those who only show up for a couple of scenes. Something with this many moving parts was always going to be far too big to function as an actual movie - that it still somehow ends up being a quite entertaining (and at times genuinely shocking) piece of blockbuster entertainment is frankly nothing short of a modern miracle.
You know the story, or at least how it starts. Thanos, the big purple guy in the chair who was first teased in the post-credits scene of Avengers Assemble some 6 years ago, has finally gotten off his ass in order to unite the Chaos Emeralds and become the most powerful being that the universe has ever known, and the only people standing in his way are... well, almost everyone that we've ever met over the last ten years of the Marvel Cinematic Universe. That's Avengers: Infinity War's main selling point - it is also quite obviously the source of all its biggest flaws.