Why No Credit for Screenwriters?

I know most people on here simply won't care, but I'm getting sick-and-tired of the lack of credit given to screenwriters. It's like their work just doesn't matter. At all. Most people just blindly follow the Film director's ass and applause him for the story of his film, despite it not even being his work. I could think of tons of great films where the director gets the credit, and the writer becomes left out of the equation. :x

Taxi Driver
Psycho
Shutter Island
Rear Window
vertigo
Jurassic Park
Seven/Se7en
Alien
Etc.

Just to name a few.

I agree. Screenwriters never get as much credit as they should. Even Steven Spielberg said once that he uses the writer's vision for his films, not his own.