What is the reason for overly sad movies like Fault/Stars?

Well, the reason they are made is that a studio or producer thought that movie could be made by it - and, it seems, they are correct. Sad movies these days, in my opinion, do not earn their tears. Take one already mentioned, "As It Is In Heaven," whose protag dies ever or ever so slowly while he listen to his choir (which metaphorically becomes his heavenly choir) sing syrupy quasi-religious music in a scene that just goes on and on and on and on and on.......... This kind of obvious manipulation has an opposite effect on me. I find these movies unbearable and unwatchable. They make me angry. Nary a tear from me. Modern directors don't know how to let a person die without dragging out the scene to infinity (it feels like) and playing it over soaring violins. The ending to "Big Fish" is another I detest.

As for why people go to see them and allow this kind of cynical and obvious manipulation work on them...well, since you describe your symptoms so vividly, maybe you can tell me. You have clearly gone to see them.

A (mostly) animated documentary from last year, "Tower," is very moving; I came very near tears - and the film earned them by telling its story in a straight-forward way, without all the deliberate and obvious tear-jerking.

Learn to recognize - and hate - cheap movie tricks to pull tears.