Because files from the build folder will contain your local URL, for example, http://localhost:1337, as it was built on the localhost with development env. If you will put your local build on the production server then you will notice that it tries to request localhost
This is why I added the build folder to gitignore in my projects. I build the frontend during deployment by using production envs with “NODE_ENV=production yarn build”
You asked how to create an admin user, so the question lead me to the idea that you don’t want to create it from the login page 
But yes, I recommend using the plugin only on local env for faster development.