There's a handful of very capable and mature admin solutions (activeadmin, rails_admin, administrate, and still people coming up with new ideas, see avo and mad

There's a handful of very capable and mature admin solutions (activeadmin, rails_admin, administrate, and still people coming up with new ideas, see avo and madmin as newer examples), they just haven't had one become bundled mainline. Similar situation with authentication, where devise has been a mainstay for over a decade, but it's not always the best fit for everybody so core rails still just has the basic plumbing to build it yourself (with the low-level has_secure_password).