Views.

It's that simple for most. Most of these recreations aren't done with much effort, if any at all. Yet, there is a lot of views and likes attached. A good amount of potential revenue.

I remade the BATMAN V SUPERMAN with a terible result to prove it, before Warner Bros cramped my style.
https://www.youtube.com/watch?v=NxK3aru7p9M
I also remade the Fantastic Four trailer earlier this year, to some modest effort on my part. Pretty decent outcome.
https://www.youtube.com/watch?v=_WQ0KOnIInk
The problem some must be having wth this is that it is a quick way to become popular on youtube with little work. There are animators that shall not be named with thousands of subs and milllions of views with terrible trailer remakes. All are trying to crack them out quickly before the rest show up.

My Film Review blog http://funmismoviereviews.blogspot.ca/
My artwork http://funmiproductions.deviantart.com/

I think this right here is the best reason for recreating a trailer in LEGO. At least in drawing, when you are learning you're often encouraged to study and even copy the works of great artists to gain a better understanding of drawing principles, and also an artist's style and so on.

But you could also do this with short movie clips. The primary reason for recreating trailers seems to be views.