Emacs is best understood not just as an editor, but as an integrated computing environment promoting user freedoms to the fullest extent possible in a manner naturally integrated with its user interface and extensibility. In that sense, it shouldn’t be the least bit surprising to see the kind of in-built application suite one would expect from an operating system or desktop environment. One must also remember that historically the most common computer (power) users were academics, and not “software developers” — so expect a lot of scratches for their itches.

It’s a bit like asking why does bash come with a utility like awk or bc; it’s equally natural.