If they were making movies for comic fans GOTG could be made to stand alone and then interact later. However for the movie audience that has never read a comic book they need to be hand held and walked through, showing a connection from the start or else they will say "How did that happen?" when the inevitable team up occurs.
Like it or not, these films have to get the audience up to speed and unfortunately that means dumbing it down to express what should be obvious. Marvel is making a big universe and everything is a vehicle to expand it further.