Because on heroku, your application is located in the /app folder (from the root / ). This is NOT the application folder of your application, which is /app/app .
File structure:
/app/app/... /app/config/.. /app/Gemfile ...
You can do an experiment.
Launch the hero console and find the file structure.
> heroku console > puts `ls` > puts `pwd`
cutalion
source share