That is in line with the Free Software and GNU philosophy. It isn't just about making the source available to you. Rather, they expect that you - the user - will tinker around and modify it substantially to suit your needs. So they include practically everything needed to achieve that - including full documentation, developer tools, code introspection capabilities and sometimes even entire textbooks (The bundled elisp book is actually an introductory textbook on programming).

The other GNU software all follow a similar strategy - be it Guix, Shepherd, Guile, Poke,... It's so pleasant to work with them. Too bad this approach didn't catch on.