The core Avengers (Captain America, Black Widow, Vision, Scarlet Witch, Falcon, War Machine) are there because the Avengers facility is now Steve's home.
Tony is there because of the Accords.
Ant-Man and Hawkeye are there because Steve recruits them to help him in his mission to defeat the super soldiers in Siberia.
T'Challa is there because he and his father are present at the ratification of the accords in Vienna and then Black Panther becomes a factor because his father is killed there.
Spider-Man is the only character you could say is truly shoe-horned in.
What I'm trying to say is, yes, it's a lot of characters, but they're all there for a purpose that revolves around Captain America or at least the Accords, which is a Captain America event. It is the ONLY story in Captain America lore that contains this many fellow Avengers and Marvel decided to tell this story to cap off an amazing trilogy. Makes sense.
It's NOT an Avengers movie. It's just a crucial chapter in the story of Captain America that happens to involve many of his fellow superheros because of the Accords. Period. The end.