Because the characters care, and they seem authentic about it (ie, it doesn't seem contrived)?
I mean maybe I'm just a cheap date, because the first marvel movie I saw was Endgame, and I enjoyed that too. But it seems to me that you don't always have to see behind the curtain to understand what the characters are doing, or why, or to empathize with what they're trying to accomplish.
Point is - I'm sure the story is more interesting to people who have seen it all, and who know the history. But the story is still accessible to the people who are new to these characters, and I think that's what matters.