Themes

Alot of movies incorporate quite similar themes and subplots. For instance the currently highly successful Marvel franchise tended to potray a classic good vs evil archetypes. Showing that in such Marvel worlds good always wins always evil. Such representation of the archetype could instill in current society that doing good for the society is a good thing ultimately resulting in a better society. Although the very the notion that society could be swayed by a random action movie they choose to watch for enjoyment may not sit that well. Although I can say with more certainty that visuals such as 9/11 could say did have an impact on society fear in the Western World while songs like https://www.youtube.com/watch?v=y6Sxv-sUYtM may of made society (atleast the pop culture music listening people) feel slightly happier.  

Another benefit of using similar themes (and plots and actually) you can actually generate interest 

in a film from inter-visual and contextual references (ie. Rambo No. 6 etc :P ). Ultimately this results in directors (and more specifically producers) seeking to go for low risk high audience movie which don't challenge their creative potential. While other films lurking in the background are often deemed arthouse or only found  by viewers after they been released (ie. for instance i think this applied for some with the move  "Shawshank Redemption" and the movie "In to the Wild" )

A more positive perspective (less financial reason) why movies share such common themes could be due to the fact that the movie themes are loosely based on quite core human experiences which humans can relate to. Therefore instead of seeing it as unoriginal movie plots it just could be different directors connecting with the same human experience/emotion/thoughts.

One potential example of this is depicted across 3 movies below. 

 Beauty and the Beast

Howl Moving Castle 

Secret Garden