TL;DR of the above 
Discourse is complex piece of software that depends on a lot of other programs/libraries. This is not a Linux specific thing. It would like asking, why can’t I install any program on Windows by simply copying a folder. It’s a bit more complicated than that. 
To answer your question more specifically, we would probably need to know to which software are you comparing here. For example, a lot of software that you can install like that is only working, because the server is already preconfigured to support it (if it is e.g. somethingn written in PHP).