It's not a complaint so much as it is an observation that yeah, watching Civil War it feels like you've actually missed out on a lot of stuff if you aren't following these films very closely. I haven't had that sensation with any of the other Marvel releases.
Plenty of people skipped Cap1 and Cap2......Civil War's worldwide box office is 3X as much as First Avenger, and nearly 2X as much as Winter Soldier.
People also rewatch films they really like. Also, movies do allusions to events or stories that happen outside the movie ALL THE TIME. James Bond does it, Indiana Jones does it, and everyone accepts that other adventures happen offscreen that they don't get to see. The only difference is that the MCU gives you the option to fully explore those side stories if you want to.