Because `make all install` would first build, then install, while `make install` would just do the installation, regardless of build state.

Wouldn't any sane install target depend on the build products it seeks to install, and consequently build before installing?

Why not have install do the build state, and then in most cases it wouldn't need to be run at all since the target would be up-to-date? Just curious.

I usually structure my targets so that I can run `make test` immediately without having to know what all the random commands are to build/install.