Why are so many characters with lines listed as "uncredited"

Rating: 10/10 (28 April 2015)

I love the film, and I love the complexity. I think the Agents of SHIELD ep just prior to the film's release needs to be seen before the film. I'm confused as to why on IMDb there are actors and actresses with lines in the film who are in the "uncredited" section. I will be watching this film dozens of times to look at ever piece that relates to the previous films, Agents of SHIELD, and upcoming stories. It would be nice if movie theaters would play the lead in (last two SHIELD episodes as a pre-film help or anyone who is lost at the beginning of the film). I'm a bit confused as to why Marvel is tying this film to ones coming out in 2017 and 2019 though,... that's very difficult to keep sorted.