For most "summer popcorn movies" - great action films - closure is very easy. At the end of the movie, the bad guys are typically dead or all arrested and put away for a very long time. (Mostly, the former).
Check out, for example, most of the Marvel movies: in the Avengers, one well-placed explosion destroys all the income alien forces. In Avengers 2, a robot AI is the enemy, and it has to be completely eradicated by the end of the film. In all of the Iron Man movies, the bad guys are dead at the end. The same holds true for most parallel movies.
Closure, in this case, is very easy. There's no need for peace talks, reconciliation, justice, redemptive conversations, and the like.
We can fall for the same temptation to closure in our own lives. Just pull out. Just write someone off, stop talking to them, don't have any further contact. Unfriend, unfollow, block, change phone numbers, move with no forwarding address. Get on a plane.
This may be part of the temptation to really poorly designed short-term mission involvement: it's an effort that eases our conscience (we've done our missions duty) while offering easy closure (just go, do two weeks, get on a plane and come back and leave it all behind).
When closure is easy, what does that say about the relationships involved?
When people are easily left, are they truly people that we care about, love, are charitable toward - or are they aliens?
When closure is difficult, it says something about our perception of the value of those involved.