I wish someone could explain why writers.....MAJOR SPOILER INCLUDED (blacked out)

Why do writers too often utilize the overused habit of creating "smart" characters that do incredibly stupid things and things that even the simplest viewer can figure out way before the character??

The suspecting daughter is friends with a troubled man who has tried to kill himself before and he even remarks how he thinks that suicide runs in the family because his father killed himself. She then gives the man a revolver she found her father had hidden, even after he pleads with her he does not want it. Then, later, he begs her to stay with him one night and more or less says to her that if she doesn't, he will kill himself and what does she do? She begs off by telling them she had to leave and will call him tomorrow. Yes, I get that this was a way to move the story forward and to the eventual ending, but it was complete BS. Just complete utterly lazy writing.