Not everyone is a beginner web designer. Some people are trying to build things from scratch using JS.
"Just use this magic build.sh!" sounds a lot like "Just use this magic starter kit!" or "Just use yeoman!"
Who is supposed to write this 'build.sh' in this scenario of yours?
Without fail, every single magic build system or magic starter project I have ever used is now deprecated and abandoned.
Makefiles? No one uses those anymore, use Grunt!
Grunt? No one uses that anymore, use Gulp!
Gulp? No one uses that anymore, use webpack!
I have a react app that I built on top of a starter kit that I now need to rebuild using create-react-app because the build process broke when I tried updating something.
Meanwhile, I can pick up a python project I worked on 10 years ago, or a go project I started 4 years ago and everything works exactly the same.