[MCU] Is there a canonical explanation as to why the Avengers simply don’t show up to protect earth?

For example, in Thor: The Dark World earth is at risk of total annihilation, yet no superhero shows up to protect it (except Thor, of course). Ironman? Captain America? SHIELD? Nobody mentions them or even tries to contact them.

In Captain America: The Winter Doldier, SHIELD turns out to be compromised by Hydra and planning world domination, extrajudicial assassination etc. I can see why this would be below Thor’s pay grade, but where the heck is Iron Man? Hulk? Even Hawkeye doesn’t seem to care enough about this.

Now, I’m not an idiot, I get that these movies are not all supposed to be “The Avengers” kind of movies, but what is the canonical explanation for how some superheroes just sometimes don’t care that humanity is at risk?